BLAH BLAH MAHOU SHOUNEN MAGICAL BOY Like the rest of the characters in the series with Guardian Charas, he can do a standard "Character Change." Nothing physically changes, but the mental differences are noticeable--with Temari, he...typically turns into a violent psychopath that wields a naginata. With Rhythm, tl;dr he just becomes a pimp he becomes ridiculously loud and outgoing and basically a giant ham. Still a pimp because he flirts with all the ladies in that state.

Slightly more complicated is the "Character Transformation" in which their personalities are more drastically altered, but they gain powers--all are more agile and able to withstand stronger physical blows. With Nagi in particular, he has two, again, one with Temari and one with Rhythm. Temari's is Yamato Maihime, which in basics is a ~*~PRETTYFUL~*~ kimono-clad lady who can manipulate ribbons in offense(this is as broad as it sounds), and razor-like cherry-blossom petals in defense and distraction. In this form he can fuse powers with Rima Mashiro's attacks into a giant attack of pink and white sparkles called "Queen's Waltz." Rhythm's is turning Nagi into a gangster called Beat Jumper, which is as Street™ as it sounds. In that form he can just throw basketballs and other projectiles of that sort that have energy trails and can explode with magic crap on contact. And he can fly because there are wings on his shoes yadda yadda.

His Guardian Charas themselves need to be present for all of these things to take place. They themselves don't have many abilities besides unlocking the Changes/Transformations. Though they can fly and they sleep in little tiny eggs. (And they're essentially facets of Nagihiko himself, so they wouldn't count as appable characters.)

Hello? ...is this working? Sorry, aheh, I'm not used to working with rotary phones still. So old-fashioned. I kind of miss my cellphone. Anyways, ah...I have a question for any of you who's listening.

If you could get out of here, would you? If there was a magical solution that would give you the option of leaving this town forever, would you take it? Even if meant forgetting everything you went through, here?

[A pause, and a laugh.] Sorry. That sounds so very ominous, doesn't it?