Hmm... If I may create a few new Sealstones, then...
Bud Spirit Synchro - Equip on a character that can use swords. During their turn, they may "Synchro" with another character, increasing the synchro'd character's status defense by 25%. If one character during this Synchro would be damaged, both the Spirit user and the character they are Synchro'd with take damage (this bypasses any effect that would force targetting to a single character). If the equipped character dies, then the effect is canceled.
Elazul Spirit Synchro - Equip on a character that can use swords. During their turn, they may "Synchro" with another character, increasing the rate of limit gain of the synchro'd character (not self) per time it is increased by 50%. If one character during this Synchro would be damaged, both the Spirit user and the character they are Synchro'd with take damage (this bypasses any effect that would force targetting to a single character). If the equipped character dies, then the effect is canceled.
Elle Spirit Synchro - Equip on any character. During their turn, they may "Synchro" with another character, decreasing the damage the synchro'd character takes from elemental attacks by 25%. If one character during this Synchro would be damaged, both the Spirit user and the character they are Synchro'd with take damage (this bypasses any effect that would force targetting to a single character). If the equipped character dies, then the effect is canceled.
Escad Spirit Synchro - Equip on a character that can use swords. During their turn, they may "Synchro" with another character, changing the defense check of the synchro'd character's physical attack to MDEF. If one character during this Synchro would be damaged, both the Spirit user and the character they are Synchro'd with take damage (this bypasses any effect that would force targetting to a single character). If the equipped character dies, then the effect is canceled.
Lisa Spirit Synchro - Equip on a character that can use staves. During their turn, they may "Synchro" with another character, increasing the Synchro'd character's MDEF by 25%. If one character during this Synchro would be damaged, both the Spirit user and the character they are Synchro'd with take damage (this bypasses any effect that would force targetting to a single character). If the equipped character dies, then the effect is canceled.
The rest... Either I don't have data on, or are incredibly useless in the dungeon, or WAY too overpowered.
Also, for character suggestion of the day...
Shania - 3.5 (SH3)
Pros: Lots of options! Damage! Possible build-up of double-actions/chained turns! Wins resource wars! Gets the ultimate buff on Floor 6 or 7!
Cons: Whatever Yuri has to deal with, without some of his more broken options. Always Darkness-elemental. Johnny makes her envious on damage on floor 7. No healing until at least floor 3 or 4.