This may seem a little overpowered, but based on what you said, if the feature was implemented, I would say that it would be looked at as follows.
IE: Rooks, using a skill called Vampiric Slash/Strike, Heals back 50% of inflicted damage, costs 10 mp.
At that point, if you need some healing, you would have to decide if you want to attack And heal back some, instead of just burning up heal spells for him. It isnt too OP and in battle makes you decide if you want to strike as well, or spam cards, or heal via heal 1/2/3 and then spam cards, etc. Of course, he gains the elemental spells later. Even at end game, him hitting for 150+ dmg with this skill and healing back 75ish HP wouldnt be too OP, since he is partially relied on to use cards where possible.
If you really wanted to make it more OP, similar to the spirit of water, you could make that skill hit for around 1.5x his attack damage, spread the healing to all party members, but drop its healing effectiveness to 20%. It would make it similar to a health top off. Oh, and increase the mana cost to 15mp. At that point, say mid game, if he hits for 80 atk dmg normally, and he uses the skill, it hits for 120 dmg, and the heal for the entire party at 20% becomes 24 hp per party member, literally 1/3 of Marids healing XD
It seems like that would make it a preferable boss attack or hardier enemies when you want to dish out more single target dmg, or just want that quick health top off while still being able to attack.
As far as an MP Drain since you said enemies act like pokemon red/blue with no PP in the game for them, well...I would say implement an item, like one of the unused rings if its doable without too much headache, and it becomes an accessory equip. Only 1 exists, found slightly later in the game but not too late, and the character who wields it just gains a flat 5 MP regeneration per turn. That would help with regaining mp at a steady pace without having to rely on a dmg formula, and also prevents late game heavy scaling, where say it healed 10% mp of physical dmg so rooks cant hit for 150-200+ phys and pop 15-20 mp back into his pocket. It also motivates you to put it on a caster so that they can alternate between Attribute spells and physical while still gaining MP in the process, but not a crap ton.
Just a couple ideas that might diversify the gameplay somewhat. I know everyone has their play style, but most of the time until end game I find myself spamming nothing but Card attacks unless the physical dmg makes more sense against the mob/boss. Oh, and i almost forgot. When a character is taken via plot, the ring is returned back to the inventory, IE: Teefa / Darwin in Bintel Castle hold off enemies for you, ones wearing the ring, and then its gone, normally i think anyway