One of the interesting facts about Relm is that she can equip more pieces (that is, distinct item types) of basic equipment than pretty much any other character. So much so, that if you do a deep dive on understanding the Merit Award (the pic fragment shows a brief, er, "sketch" of this), her native equipping ability is a useful starting point to understand which items each character CANNOT equip, even if they have the Merit Award on. I may not be phrasing it quite right but that's about the idea.

Basically there's a number of novelty items and "cute outfits", many of which are in fact good equipment, which she can equip (chocobo suit, Moogle suit, that sort of thing), but once you've gone around in the Veldt a few times you'll work up some Behemoth Suits, and that's the killer app armor for both Strago and Relm.

As the youngest team member, Relm also in fact has (IIRC) both the highest max MP and magic power for a given level, even higher than Celes or Terra. So a player focusing on magic boosting in the middlegame is well served to start with her.

The primary utility of Sketch/Control is not for direct help in battle, but rather as a gimmicky way to teach Strago lores or as a creative way to immobilize enemies to farm rare equipment. Happily, once you get an enemy alone, there's no TIME LIMIT on how long they are controlled. So you can neutralize a single target and just keep pot-shotting steals or morphs until the thing you want actually pops. The fact that Relm's ability helps Strago is another clue (along with their general costumes/stature) that the two form a unit. They naturally work well together.