He is used for his leader skill "Resolve", which allows you to live in case which you should've died, IF you have more than 70% of your total HP before the enemy attacked, that is.
(ie. you have 800/1000HP, enemy attack is 9999, you live)
However, if say one enemy got your HP to below 70%, and then another enemy attacked again, Resolve will not activate should you suppose to be killed.
You don't want to level this guy, keep him as low level as possible.
: (weird name in US version...)
Its spec is 10 HP/5 ATK/80 RCU, incredibly low, but it's exactly what we needed - since we want our HP to be as low as possible, and with enemies' DEF being so high (4000~60000+)
, there's no need to stack for high ATK, while 80 RCU is a nice plus.
You can also use other evolution material monsters like Water/Grass masks, or goblins, or what not, but those usually have higher HP with shitty RCU, making it that sometimes you might not have the time to regain back to 70%+ HP when you needed it. I'd personally suggest only use other water monster if you couldn't gather 4 of these in time before the dungeon expires. (then again, you can do it again next week!)
Friend Leader - Preferably Siren
Siren's leader ability heals you as long as you did orb elimination that turn, her 1st and 2nd stage heal 100% of their own RCU amount (RCU number from other 5 monsters you're using will not be counted in)
, while her 3rd stage heals for 300% instead of 100%.
If you can get a 3rd stage of Siren from your buddy, great, if not, 1st and 2nd are fine too. The only difference is that for the later you might have to do orb elimination for several turns. (again, why Spirit is awesome with its low HP for you to heal back to 70%+ quickly)
Friend Leader - Other choices
1. Grass/Light Healer Girl
Same type of leader skill with Siren, if you could not make the Water/Grass team but have 4 Light Spirit instead, you can considering using her. However they both have cons
: Grass's 1st and 2nd stage only heals 50% of her own RCU amount, unlike Siren's 100% heal. Also there is no Light Orge, so this means whenever you do a Light combo, you miss out 1 attack compare to the Water/Grass team. (somewhat crucial, explained later)
2. Poison monster
Such as Succubus or Neptune the Water Norse God, they'd save you the trouble when you entered the 5th floor to kill all the monsters including Mystic Mask instantly, but more risky for you to survive on the previous 4 floors.
Of course, you also lose out the convenient auto-heal by using them instead. But if your RCU is enough and you're confident that you can always use the heal orb to manually get back to 70% HP, then sure go for it.
That's right, anything. If you want challenge or is confident about your skill in orb elimination and combo, you can totally use any monster, even if it's not the same element type to the rest of your team. (just don't get your HP too high if your RCU is not good enough to heal back in short time)