Leo is useless

[Rez09]

It is particularly difficult to judge Leo as a character due both to his short stint as a playable character, and the often counter-intuitive nature of character damage in the game.

As he is, I find Leo an unfortunately underwhelming character. Despite the slight buildup the game does for him, his stats are not terribly impressive, he deals less damage than the majority of the party members he replaces, and his class ability is far from stellar in the state he is found in. Additionally, the Offering is a relatively poor relic on its own, roughly on par with the Genji Glove without the defense penalty, and the majority of its power comes from pairing it with select other items, none of which Leo possesses. In complete honesty, everything he does the vast majority of the cast can do better, potentially -- in the case of Gau, Edgar, Sabin, and Shadow -- at the same time you have access to him.

That said, if he were a fully playable character, he would be disgustingly overpowered in a game already trivialized by its powerful cast and item combinations. Much like most of the cast (but Sabin and Cyan in particular), even when strength appears to be the best stat to raise, magic is actually the ideal stat to cap; on Leo it would allow him to capitalize on Shock's 128 spell power (22 less than Ultima), 0 cast time, and 0 mana cost. Due to his crystal sword it can be assumed he has access to the same weapons as Celes and Terra, which means he can use the Lightbringer and Ultima Weapon without a Merit Award, both of which synergize with his built-in Offering, and the former benefiting from high magic build characters as well.

So, Leo's pretty sub par as you get him, but if he (and his free Ultima) joined as a permanent member, he'd be a contender for most powerful character in the game.