@IsaacBai This is the number one thing i kept thinking while reading the manga. But given the fact that no one noticed that there was something off about the MC everytime he time travelled back and forth im guessing his "kid" personality is identical to his adult personality apart from the fact that he knows about time travelling.
My other theory is that every time he time travels he simply transfers his memories. Meaning that after the first time travel adult MC is actually living his entire life as his kid version, but when he travels back to the future he doesn't travel he simply lives his life out as his kid version up until the moment in the future where he simply overwrites his memories with the ones he had during the handshake.
@IsaacBai I thought it was weird too. Especially when he proposed to what, 16 y/o Hana when he was an adult in a kid's body? I did like fanghue's theories tho