It is definitely worth mlbing healers. The heal makes a huge difference in amount healed when you get higher hp. A 5% change can make as big of a diference as 3k hp and mlbing a healer makes a 15% difference. If it's a 4 star, it's easy to lb but 5 star you might want to hold off for a bit if you haven't lb'd anything else. Of course the 1st 5 star I mlb'd was a healer (maid Kotori) since I only want to really lb Kotori (Kotori best grill) lol. Anyways there are all sorts of good cards as a healer for Kotori but you should include at least one attacker. Good healers are 4 star run3 single heal (good for ez heal though I don't like depending on a run too often since I mainly use odd and even), 2x3 revive and heal 4 star (seriously amazing; when I go offense Kotori, I still use the revive one. It heals 50% at mlb which is great). 4 star 2x2 wedding Kotori which has heal over 3 turns (it's not amazing but healing over several turns is nice in its own way. 2x2 is the eziest requirement as well.) if you don't have any other heals though you would have to use 3 star 2x2 heal which is pretty bad but ez to use. I don't recommend using the 3 star even 3 1turn 45% reduce if you have the choice since your stats take a huge dip and you basically skip a turn just to reduce a little bit of dmg while taking up a slot. I personally already feel there aren't enough slots to put in all the good Kotoris so I can't put that in when I can just use her to heal or do dmg instead. Of the 5 stars, there are mainly only attackers and the maid Kotori is the only healer. She is good though since she has 2+3 aoe heal of 45% at mlb: when I don't play an aggressive team, I'd use her and revive for the heals. Anyways that's about it.
UPDATE: @Tamamo-no-Bae I can't read it either but the 2x3 4 star with a heal is the revive.
@Jellal Oh god it's a life steal? 50%? I never thought it to be life steal since there's such a high %. That's pretty ridiculous lol.