There’s a discussion amongst the remaining members of the Six Heroes after this event.
1. The results of minute details found at investigating the house of the kidnappers leads to the conclusion that Reid might be alive again, reincarnated as a child. Due to the fact that they know how Reid does things. They know about reincarnation magic, Maxwell also seems able to use it but supposedly the randomness factor of reincarnation magic makes it less ideal to use.
2. Since they believe Reid is alive again, they wonder why he never tried to get in touch with them.
3. They at first were going to try and find him. But Nicole somehow stopped that.
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Later on after a different incident involving Cortina, Maxwell correctly infers Nicole is Reid due to the events. He also manages to somewhat trick Nicole in answering to his old name. This proving that Nicole is Reid.
At this point in the story, through magic, Nicole is able to project her old appearance. And through an even better magic, she can create an older version of herself. Nicole also gets Reid’s gauntlets and changes them since the original form would be instant evidence she is Reid since they are one of a kind gauntlets.
Based on LN pictures, the outfit she is forced to wear in adult form is based on Maxwell’s preferences and shows lots of skin. Also Maxwell admits that if he was 100 years younger, he would push down Nicole(adult mode) and make her his wife. Nicole/Reid shivers at that thought.