manga spoilers
tentative theory
All of this is going on in Tsubasa’s mind as he’s slipping in and out of coma. His brain is trying to process everything that’s happened and reorganize itself.
Kakaeru 1 has a secret family. Tsubasa figured this out and told Minako at some point. Kakeru 2 is either his mom’s boyfriend or perhaps an adult outside the family that Tsubasa knows who he sees as a possible paternal figure (probably a teacher or neighbor).
What’s interesting is before Kakeru 2 joined the family there were six people in the family, but now there are seven, which makes the general fandom theory that they all represent one of the deadly sins likely. So far I’m thinking it’s - Tsubasa - wrath, Shiroi - gluttony, Minako - sloth, and Kaker 1 - lust. So that leaves us with pride, envy, and greed. My guess is the grandparents are pride and greed and Kakeru 2 will be envy possibly since he suddenly appears to take Kakeru 1′s place like maybe he’s envious of what Kakeru 1 had.
Should be interesting to see how this evolves as we gain more information. I’m also curious if everyone actually does have amnesia or if this is just how Tsubasa’s brain is coping with the situation as it tries to piece things back together again.