I think in terms of personality, Celestia is perhaps closest to one of these guys:

The War Doctor

This guy did horrible things during the Time War, but in the name of Peace, and Sanity, and believe you me, there was still regret. There wouldn't have been either if Celestia didn't lock her sister in the moon! Celestia also felt that it was still a mistake, what she did, even though it was what was necessary.

The Ninth Doctor

The Ninth Doctor would oscillate between sassy and stern, which we see in her sometimes. He was angry at himself for his past life, and he was also a master planner, similar to how Celestia had planned to gradually condition Twilight to replace her because she felt that Twilight would be a better ruler than she.

The Tenth Doctor

The Tenth Doctor would wear a smile of joy and laughter to hide his sorrow and sadness, which might explain why Celestia likes to tea-troll, and sometimes acted like fun incarnate.