Sounds good so far. I was worried about the first three characters being so close in age, but it looks like things are going to be plenty diverse. Hopefully there will be no stabbing early on because [s]I'm still working out how Amie's sword and powers work[/s] that would be an awkward start.

Perhaps something akin to the Monado Arts (Xenoblade Chronicles) that are used by Shulk in Super Smash Bros. The sword cycles between five stat buffs (jump height, movement speed, shield, attack damage, and knockback) that last for a brief time. It could be practical outside of combat as well. Better still, something on her person (a glove, for example) could cycle through the abilities and apply them to the sword because future science.

Here's a link and description of the Arts. 
[url]https://www.ssbwiki.com/Monado_Arts[/url]