It doesn't really show, but he IS a tiefling. He might lack the horns but he makes up for it with pointed teeth, a forked tongue,
black patches on his skin and a couple of other things which will be revealed in private.
Dearleigh, or Leigh, or 'Mirage', even, is a Hexblade warlock, in pact with a sword (or perhaps a currently sword-shaped being?) which spent
an unhealthy amount of time in the Shadowfell and got corrupted by it. Not to an extent that it'd be dangerous for his owner; quite the contrary.
The sword is actively holding off a spell that was placed on Leigh. It should have killed him, sapping his life but instead, it aged him rapidly.
Hence, Leigh is looking for a cure for both his and his bladed companion's afflictions.
The accelerated aging only affected the body, not the mind, so poor Leigh spent most of his teenage years locked in the house, since it is
really hard to make friends when those of your age are put off by how you look and those who look like you are put off by your age.
He found solace in music; he became quite a skilled musician and dancer. This affects both his spellcasting and his fighting style.
Leigh's family, which is decently well-to-do, did everything to help him, and he is grateful to them. But enough is enough; he wants to
reclaim his lost years and restore his sword to its former glory.