First, let's list the characters from the show, and how they change in the multiverse.

Celestia- In the Main Eclipseverse, she has ruled alongside her sister Luna for 2000 years. Together, they have guided Equestria to be slightly more advanced than portrayed in the show, but not by much. However, due to their combined rule, the military no longer uses midevil weaponry, and instead has more American Revolution style weaponry. In the Scorch-Frost Eclipseverse, Celestia fell into darkness alongside her sister 2000 years ago, and was imprisoned in the sun as Burning Sun. Doesn't exist in Tyrant Eclipseverse.

Luna- In the Main Eclipseverse, Luna never became jealous of Celestia, because the ponies loved them both equally, and needed their Night Princess. Being alongside her sister has allowed her to have somepony to talk to over the long, lonely years. In the Scorch-Frost Eclipseverse, Luna fell into Darkness alongside her sister 2000 years ago, and was imprisoned in the moon as Chilling Moon. Doesn't exist in Tyrant Eclipseverse.

Twilight Sparkle Astrolight- She never became the student of royalty in either reality, and instead spends most of her time either with her brother or her only friend, Moondancer. She, in the Scorch-Frost Eclipseverse, even accepted Moondancer's offer to go to a party, stating that "it might be a good learning experience." Never got her Cutie Mark in Tyrant Eclipseverse.

Rainbow Dash- Stuck in a dead-end job as Cloudsdale's weather captain. Never got her Cutie Mark in Tyrant Eclipseverse.

Applejack Apple- Leader of the Ponyville Farmer's Union. This has taught her a valuable lesson in cooperation. Also, she doesn't fear Zecora due to her Granny trading goods with her every so often. Never got her Cutie Mark in Tyrant Eclipseverse.

Rarity Belle- Not much is different, save for the fact that she is open to business partnerships. She has a partnership with Roseluck's mother's flower shop, which likes to sell flowers as accessories sometimes. Never got her Cutie Mark in Tyrant Eclipseverse.

Fluttershy- Still tends to be a recluse, but friendly with some of the local vendors. In Tyrant Eclipseverse, she was born a thestral and is called Flutterbat. She is one of the six "Marked Ones" that are thought to be able to wield the Elements of Harmony to save Equestria.

Pinkie- Still Pinkie- except in the Tyrant Eclipseverse, where she never got her Cutie Mark.

Shining Armor Astrolight- Head of Equestria's Royal Guard, which are not as much of a joke as in the show.

Mayor Ivory Scrolls- More tolerant to less prominent pony tribes, as well as non-ponies. Never got her Cutie Mark in Tyrant Eclipseverse.

Trixie D'Glamour Lulamoon- While she hasn't made an appearance yet in Main or Scorch-Frost, we can infer something- she is an heir to the Noble House Lulamoon. In Tyrant Eclipseverse, she is a Shift-blood unicorn that is one of the six "Marked Ones" that are thought to be able to wield the Elements of Harmony to save Equestria.

Lightning Dust- In Tyrant Eclipseverse, she is one of the six "Marked Ones" that are thought to be able to wield the Elements of Harmony to save Equestria.

Octavia Melody- In Tyrant Eclipseverse, she is one of the six "Marked Ones" that are thought to be able to wield the Elements of Harmony to save Equestria.

Moondancer- Friend of Twilight's. In Tyrant Eclipseverse, she is one of the six "Marked Ones" that are thought to be able to wield the Elements of Harmony to save Equestria.

Screwball- Discord's daughter. In Tyrant Eclipseverse, she is one of the six "Marked Ones" that are thought to be able to wield the Elements of Harmony to save Equestria.

Roseluck Flora- Main Eclipseverse Element of Charity. Her mother Violet instilled into her a giving nature from a young age. She obtained her Cutie Mark at the same time as her sisters, Daisy and Lily. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Cloudkicker- Main Eclipseverse Element of Compassion. She has always striven to be kind to all she meets- no matter how horrible they act. She is Ponyville's weather captain. She obtained her Cutie Mark on a particularly stormy day. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Zecora- Main Eclipseverse Element of Honor. It is tradition among her tribe to teach all the young ones from an early age to be up front and honest, and to never go back on your word. She obtained her Cutie Mark in her homeland of Zebrica. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Starlight Glimmer- Scorch-Frost Eclipseverse Element of Magic. In the Scorch-Frost Eclipseverse, she never founded "Our Town," and instead became King Eclipse's personal student. How she obtained her Cutie Mark is unknown. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Berry Punch- Scorch-Frost Eclipseverse Element of Humor. She lives to make ponies happy, using her brewery skills to read a pony's mood. She obtained her Cutie Mark during a cider-making contest, which she won first prize. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Sunset Shimmer- Second Personal student of King Eclipse, she is on Probation in Hayburg. Unknown where she is in the Main Eclipseverse. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Now, let's take a look at the OCs the Multiverse has to offer.

Eclipse- In the Main Eclipseverse, he was sealed away 2000 years ago by his daughters in the "spirit" of an eclipse as Dark King Eclipse. In the Scorch-Frost Eclipseverse, he rules Equestria alone, but with his insight, he saw that it wasn't fitting to hold the ponies back technologically. So, the ponies he rules are far closer to modern-day than the show, roughly being in the early age of the car. In Tyrant Eclipseverse, he is the Dark Tyrant, Eclipse.

Melia Breeze- Main Eclipseverse Element of Devotion. Her talent is dealing with burly storm clouds, ones that are extremely volatile. She will not abandon those she cares for, willing to risk almost everything to ensure their safety. She obtained her Cutie Mark on a dark, stormy night. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Thunder Blitz- Scorch-Frost Eclipseverse Element of Devotion. Her talent is the Thunder Boom. She cannot stand traitors, and would not betray a friend if it would save her life. She obtained her Cutie Mark after using her Thunder Boom to win a race. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Cherry Cobbler- Scorch-Frost Eclipseverse Element of Honor. Her talent is in making cherry confections, especially her namesake. She is a mare of principles, and will stop at nothing to keep a promise. She obtained her Cutie Mark while baking with her mother when she was young. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Crescendo Dream- Scorch-Frost Eclipseverse Element of Compassion. Her talent is in conducting music. She may not have a ton of patience, but she tries to reign it in, and be civil. She obtained her Cutie Mark when she was in a private music school, while subbing for the conductor. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Serenity- Scorch-Frost Eclipseverse Element of Charity. Her talent is perfume making. She always tries to be calm, although talking about nobles tends to set her off. She obtained her Cutie Mark while minding her mother's aroma shop when she was young. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

Concerta Whimsey- Main Eclipseverse Element of Humor. Her talent is her ability to sing in any octave she wishes, melding with the music perfectly. She loves using her music to cheer others up, but is not immune to being down herself. She obtained her Cutie Mark while listening to a wonderful traditional Crystal Pony song that had been passed down for generations, but the lyrics had been lost. In Tyrant Eclipseverse, she didn't get her Cutie Mark.

"Janice Dazzle"- Main Eclipseverse Element of Magic. Her talent is crystalmancy and metallurgy. She tries to reach out to everyone she meets, but keeps many secrets close to her chest. A tragic event involving her half-sister Servo has caused her to have trouble trusting others. She obtained her Cutie Mark at an absurdly young age, and she is the youngest of the group. Doesn't exist in Tyrant Eclipseverse.

"Chessmaster"- Former/Current Chess Chapmpion of Equestria. His talent is manipulation and strategy. In the Main Eclipseverse, he is missing. In the Scorch-Frost Eclipseverse, he still takes challengers to his title. He keeps many secrets close to his chest, due to him losing the ability to trust others easily. He obtained his Cutie Mark at an absurdly young age. Doesn't exist in Tyrant Eclipseverse.

"Servo"- Lost Half-Sister to Janice, Chessmaster, Maestro, Aurora, and Story. Her talent was circuitry manipulation. She died some time ago, though there have been claims of some sightings, none of which were confirmed, until recently in the Scorch-Frost Eclipseverse. She obtained her Cutie Mark at an absurdly young age. Doesn't exist in Tyrant Eclipseverse.

"Maestro Spectrum"- The Apex of Art, the Maestro of Music, the Living Spectrum, Main and Scorch-Frost Eclipseverse Element of Empowerment. His talent is art and enchantment. Seems to be privy to strange knowledge of things most do not know about. Travels endlessly across the continent, not stopping for a second. He is loved by all, and has connections to many across Mytharia. Has many secrets, which he will share with those he believes have earned his trust. He obtained his Cutie Mark at an absurdly young age. Doesn't exist in Tyrant Eclipseverse... Or does he?

"Aurora Gale"- Maestro's younger sister. She obtained her Cutie Mark at an absurdly young age. Doesn't exist in Tyrant Eclipseverse.

"Story Image"- Maestro's younger brother. He obtained his Cutie Mark at an absurdly young age. Doesn't exist in Tyrant Eclipseverse.

"Cold Heart"- The main villain manipulating the events of the Main Elipseverse. His plans are supposedly for the good of Equestria, but that can't be right. Why does he have Chessmaster's locket anyway? His origins are unknown, and his talent an enigma. Just what exactly are his plans, anyway?

"Tempered Steel"- Father of the enigma five, he has been dead for quite a while. Once was a notable blacksmith.

Nimbus Speedfang- Mother of Maestro, Story, and Aurora. Died before Story and Aurora could get to know her. Once was the fastest thestral alive.

Cloaked Figure (Revived Servo)- The orchestrator of the return of Burning Sun and Chilling Moon in the Scorch-Frost Eclipseverse. Why did she decide to test the seven heroes? Did she hope for them to fail? Succeed? Does she know something they do not? And what is that Obsidian Snowflake she pulled out?

Rubeus Obsidian Diamond- Late head of the Trigem Noble House. To the public, he is thought to be the last Trigem. However, there are rumors of five more... Died earlier in the Tyrant Eclipseverse.

Topaz Obsidian Ruby- Eldest living Trigem, and head of the Trigem Noble House in the Scorch-Frost Eclipseverse. Has disappeared in the Main Eclipseverse. Half-griffon. Doesn't exist in Tyrant Eclipseverse.

Sapphiron Rubeus Emerald- Current head of the Trigem Noble House in the Main Eclipseverse until Topaz can be found. Half-thestral. Doesn't exist in Tyrant Eclipseverse... Or does he?

Papiracha Peridot Rutile- Younger sister of Sapphiron. Doesn't exist in Tyrant Eclipseverse.

Jasper Amatrine Nephrite- Younger brother of Sapphiron. Doesn't exist in Tyrant Eclipseverse.

Lapis Opal Amethyst- Youngest living Trigem. Half-dragon. Doesn't exist in Tyrant Eclipseverse.

Tourmaline Covellite Garnet- Sixth of the children of Rubeus Obsidian Diamond, died when she was young. If she was still alive, she would be older than Topaz. Half-Minotaur. Doesn't exist in Tyrant Eclipseverse.

"Moonrise Mirage"- Not much is known about her. Obtained her Cutie Mark at a young age. Seems to have some connection to Maestro. Doesn't exist in Tyrant Eclipseverse.

Zircona Onyx Quartz- Tenth cousin of Sapphiron. Doesn't exist in Tyrant Eclipseverse.

Gearbox- An eccentric Scientist that pushes boundaries, and an old friend of Maestro's. Obtained his Cutie Mark while tinkering with machines in a factory. Didn't earn his Cutie Mark in Tyrant Eclipseverse.

Phantom Howler/Wind Shard- A necromancer that seems to have some investment in bringing back "His Great Coldness." Obtained his Cutie Mark after etching runes into his body that let him summon the dead. Doesn't exist in Tyrant Eclipseverse.

Prism- Adoptive dragon daughter of Maestro. doesn't exist in Tyrant Eclipseverse.

Monochrome- A sketch artist that is not very well known, but this doesn't bother her much. She doesn't do well with casual conversations most of the time, opting instead to try and keep to herself business-like. She is very sensitive about her work, but tries her best to keep her emotions in check. If she is overwhelmed by emotion, she will find the nearest place away from society and lose herself in her work to try and avoid thinking on them. She makes barely enough to make ends meet, living in a low end apartment in Manehattan.
Looks:
Light gray furred pegasus mare with green eyes. Has a short slate gray mane, with matching tail. Her Cutie Mark is a quill against a blank canvas.
Quirks etc:
Has a secret encyclopedic knowledge about the history of the quill pen. She moved from a "place far away", but never talks about her past outside of that.