/! Spoiler text /!
About last chapter...
The last revelation about Guren.
I was thought about Mikaela story when his mom asked him to jump out a car 120km/h I'm sure she knows he is a Seraph so I guess it mean he can't die. I think he can't die if he is a Seraph as Mika jumped and he doesn't die. So I guess Mika's parents and Yuu's parents maybe have a duty to kill their child for science maybe or something else as don't risk the curse to kill every adults.
So the second theory is someone in Guren's squad is the first Seraph, maybe all are they because they revive. But I think the first is between them.
Third theory Mirai dies for use her power. As Yuu when he transforms into king salt.
Maybe Yuu and Mika were dead before
turn into Seraph..
So conclusion : Seraph can't die.
The first Seraph is in Guren's Squad.
Ps: I don't already read Guren's light novel so maybe more informations.
Care it's my theory not the truth! And you're free to comment it