Maybe Character 1 got hit badly and just wants to react to their character getting hit, for RP dramas sake, even if no attack happens. That's fine too! Again, just reply directly to the monster post with the injury in question.
Maybe a fourth character comes in and wants to heal character 1 or just comfort them, or whatever kind of planned RP thing that might happen here. That's fine too! In this case, reply directly to the character you wish to interact with, and you can thread as usual.
IF YOU ARE A HEALER: You can heal mid-battle, but neither you or the person you heal can attack in that round, and you may only heal one person per round. (You also open your character up for attack in any round you heal in, so be careful!)
ROUND ONE
CHARACTER 1 ATTACK
CHARACTER 2 ATTACK
ROUND 2
CHARACTER 3 ATTACK
CHARACTER 1 REACTION
CHARACTER 4 REACTION
IF YOU ARE A HEALER: You can heal mid-battle, but neither you or the person you heal can attack in that round, and you may only heal one person per round. (You also open your character up for attack in any round you heal in, so be careful!)
CHARACTER 2 ATTACK