When Dean left Lisa she was pregnant. and when her memories were taken of dean and she had the child she had no idea of who the father was and gave it up for adoption. That child was found by Claudia Stilinski she couldn't leave that boy there. so she asked her husband and he agreed. considering they were having infertility probables and really wanted kids. they adopted that boy who's name was Thomas and no last name. they changed the name to Mieczysław Thomas Silinski and they called him mischief and stiles . who later falls for Derek Hale but later he get a visit from someone on his dad side of the family.