MyLittleBrother
He's finally about to reveal himself to Lia! Hopefully he doesn't use the slave mark. Plus if his partner was Helia he could just tell her that the fruit is gradually awakening all affinities for him. But then again the Leon guise wouldn't make sense especially if she finds out he became an adventurer before he ate the fruit.
using the slave mark would make the most sense here regardless of others opinions. that would allow him to let he know everything, including system and otherworld status, without fear of it 'leaking' from her, especially to her family. if she is going to be a true party member that could be crucial to keeping them both alive in a dangerous situation if she understands what he is truly capable of. blind trust at this point would not make any sense as it's not like he actually knows her. just make sure the mark can be annulled later once trust is there.
Honestly, even if the whole works knows he's from another world, this cant restrict him, as long as he has the system, he can be strong where ever there are monsters to kill, all he needs is magic points to by resources from system shop. The only problem is that he lacks magic knowledge and he also doesnt apply basic wisdom in magic. He only does what he is taught, and doesn't explore things himself.