Ditto is the redeemed main villain from "the story of Ditto".
He is a old alicorn, who many years ago (same year Luna became nightmare moon) was Celestia's most trusted student, same role Twilight 'currently' has. It's considered that Celestia cared deeply for him, as she was clearly heartbroken when he he blaimed Celestia for the death of his family, and ran away brfore Celestia had the chance to talk about it.
Years later, Ditto runs into Queen Chrysalis after her defeat in Canterlot Wedding. The Queen, after hearing how lonely he is, offers him to join her minions, and begins uncharacteristically treating him as a close friend. And even makes him her lead commander.
More years later (Twilight is now a princess). Ditto, under the belief that Celestia betrayed him, helps Chrysalis in the kidnapping of various ponies, including a loyal guard named Ganger, so the kidnapped victims can become Changelings against their will. Eventually they also capture Twilight, but this leads to Chrysalis having no choice but to fire him. As he ends up taking it too far..
A few months later he takes revenge on the main six when he kidnaps Scootaloo, AppleBloom, Sweetie Belle and Spike joining a camping trip (joining witch Scootaloo ends up becoming sympathetic after hearing about the loss of his family, and feeling she could relate, as her mother is never around anymore). But Ditto's plans get foiled and he ends up in a fist fight against the enraged Rainbow Dash. He wins but gets captured by the others.
While trying to figure out what to do with him, the girls take him to Canterlot. Where he is reconsized by Celestia. Who is heartbrokenl about what he has become.
Celestia believes Ditto can still be changed for the better. And he is taken to Ponyville to be reformed.
He dose, but mostly do to the support of Scootaloo, who he develops an brother/sister relationship with..
But unfortantly, Scootaloo's relationship with him becomes discovered by some fired Changelings wanting revenge on Ditto. And after an intense fight with Rainbow Dash and Spike (both protecting Scootaloo with their lives), Scootaloo ends kidnapped by, the now villainous, Ganger. Witch angers and horrifies Rainbow Dash, leaving her stressed out and probably heartbroken.
Fortantluuy, Ditto ends up saving Scootaloo in the end, and in doing so, Dash. Who never forgave him for the scen in the forest. Finally trusts him and would call him a friend..
Ditto is orginally shown as cruel, violent and mostly angry. But it's not who he is, it's who he's become.
Before he was redeemed, Ditto showed several levels of Schadenfreude (Sadist), as he mocks Twilight's fear and uncomfort of being stuck in a cocoon inside a disturbing room. Along with anxiously asking Chrysalis if he psyically harm one Pinkie or the others, along with enjoying Pinkie's begs, though she has a strange response towards it. He also showed anger towards Scootaloo the first time she showed pity towards him.
But it's clear that he still has a soft side, espically with Scootaloo. And he proves to be extremely loyal to her, and treats her like a little sister.
His level of careness towards the main six can often be unclear of how he truly feels about them. When first meeting Twilight, there are many moments were he refers to her as "hot", but since this is never brought up joining his redeeming, he was most likely being sarcastic, and dosen't ACTUALLY find her attractive. With Rainbow Dash, he was always clear of her untrust of him, and they rarely spoke to each other.
- It's provided that Ditto is very old, probably in his hundreds, but being an Alicorns make him look young.
- It's unknown if Ditto has ever truly forgiven Celestia. But he seems to be friends with her again.
- Ditto appears briefly in Saten Twist Adventures, possibly one of Equestria's leaders and the only known male one.