Homebrew Borderline Tiefling Species Details

Tiefling can be born to parents that look completely normal for their race(usually human), no trace of their fiendish heritage is apparent. sometimes for a few generations nothing will seem amiss, but it is a trait with a great tendency for atavism, that is it will reappear randomly (or possibly due to Reasons as some occultists imagine) and a child will be born a full on Tiefling with distinctively devilish features like red skin, yellow cats eyes, horns, a tail, sharp teeth.
However just as often that fiendish heritage will manifest in far more subtle ways and that brings us to you. You maybe just have an an unnatural shade of hair, or horns, or odd eyes, or weirdly sharp teeth, you are unlikely to have the full skin hue shift. your traits can be hidden, or written off as something else. you will probably know why they are there but you have the privilege of passing better in normal society than a full Tiefling would. Chances are pretty good that you know that you are related to some Tieflings, maybe you have a sibling that is a Tiefling or know of some ancestor that was a Tiefling.
you miss out on most of the bonuses a Tiefling would enjoy and have beginning stats similar to a Variant Human.

Borderline Tiefling Traits

chose a feat and one stat which you increase by two.

Fiendish Soul

A fairly hidden feature of your infernal heritage manifests in the very nature of your soul and it's ability to resist some magical effects. add your proficiency bonus to wisdom and charisma saving throws. You also have advantage on saving throws against charm or possession type magical effects specifically from Fiends. Who will then sense that failure being due to being a tiny bit fiendish yourself. (cue the "Not So Different" villain remark)

Ability Score Increase

Choose an Ability Score to increase by 2.

Feat

Feat

Darkvision

Thanks to your infernal heritage, you have some limited vision in dark and dim conditions. You can see in dim light within 10 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.