MOMO On-Screen!
Welcome to quite possibly the most pointless argument on the planet: why MOMO should be your on-screen character! Take this with a grain of salt though, as it really doesn't matter who you use to be honest.
1. Mintia: The Mintia mini-boss can only be initiated if you have MOMO in your party. If you already have her on-screen, then you're good to go!
2. Transformation Armors: This is something in Episode I that everyone would miss if MOMO wasn't on-screen without reading GameFAQs because they only appear when MOMO is on-screen! You won't see a flashing light in Cathedral Ship and The Song of Nephilim if you use any other character and then you wouldn't be able to properly say that you completed Episode I 100%.
3. GS Campaigns: In Episode II, you can't initiate the Our Treasure owner's GS campaign without having either MOMO or Jr. as your on-screen character because she won't talk to adults!
4. Ignored: After the Y-Data was removed in Episode II, MOMO starts to get ignored. It's especially apparent that she has a lot less time in cutscenes in Episode III, so give her some time to shine since Monolith Soft does not!
5. Power of Love: Out of all the characters' destructive abilities in the game, it's the funniest to see MOMO's! In Episode I, you see a little girl blasting stuff to smitherenes. In Episode II and III, you see the power of love as hearts/pink stuff destroy everything for you!
And there you have it! 5 reasons, albeit somewhat fallacious, to choose MOMO to lead your party!