i honestly think Sheril and Tyki come from a past of poverty and misery before knowing Mana and becoming respectively the Noah of Desires and the Noah of Pleasure. this could explain the behavior of the two brothers:
he doesn’t like anything that looks dirty or poor, but instead tends to have an love and obsession with wealth and luxury
his obsessed and compulsive behavior could be due to the miserable conditions he and his brother were in before meeting Mana and receiving a life and education that he deems ‘worthy’
his being bothered by Tyki and Wisely because of their bum look because he hates his miserable past life so he tends to hate anything that might remind him of it and tends to love Road because of her refined look accordingly
he’s quite the opposite of his brother, he doesn’t like wealth and luxury, but instead tends to be barbaric as we see in his human form
his bum behavior may be due of his ways in the miserable life he and his brother were in that never left him, even after receiving an education from Mana
however unlike his brother he doesn’t mind and dosen’t deny the miserable life and tends not to be comfortable with the noble life, but instead developed a love and care for Eeez, Clark and Momo
something else that made me think that Sheril and Tyki were poor and miserable in the past is Sheril admits he ‘loves the noble life’ while Tyki doesn’t seem used to all that luxury at all. Sheril probably finds happiness in a noble life because of all it can offer, unlike Tyki that looks like a koi out of its environment
i have some theories regarding Sheril and Tyki’s brotherly bond:
Sheril and Tyki are half brothers
Sheril dissociated himself from his original surname (Mikk) as a way to forget his past but to which Tyki is still somehow attached
it seems clear that Sheril and Tyki are quite different despite them being brothers. but i would also like to mention that Sheril represents 'Noah’s Desires’ and Tyki represents 'Noah’s Pleasure’, which i honestly think are very suitable for these two brothers:
’desire’ could represent the desire to have something like: a good life, a home, a wife and children
’pleasure’ could represent the pleasure of living and enjoying life however you like, be free to choose and pleasure yourself
which mirrors Sheril and Tyki respectively