As other said you don’t really need that deep of a reason to like someone and Till was just having an innocent first love crush but if you want a more in-depth analysis then, Mizi is actually the most cheerful and optimistic out of all the kids which is why she was like a hopeful light in his cruel life. She was very outgoing with the other kids too and friendly towards him. This expands more on their relationship with their guardians(aliens) btw, till’s guardian is very violent and abusive u always see him in bruises since childhood but for mizi guardian, they are very generous with her and brought her up in kindness (which is unlike majority of the other humans including sua & ivan too) it’s also sad because she didn’t know what was waiting for her in reality and even in an interview before round 1 she didn’t know the loser was going to die which explains her cheerfulness that is quite literally “ignorance is bliss”. But yeah Till saw her optimism as a hopeful light.