Please find below a list of Y adjectives which might be used to describe a person.
Positive
youthful
Negative
yellow-bellied
yokelish
yucky
Other
yappy
yeasty
yellow
yellowish
yielding
young
younger
youngest
youngish
yummy
There are many more lists of adjectives on this website. For more adjectives starting in Y, see this post containing a large number of adjectives beginning with the letter Y, some of which can be used for describing people.
Did I miss any adjectives? Please add them here. Thanks.