Thomas/Tony Vandom is Will's biological father.
in the comics he is a businessman who suffers from a ruthless win-or-lose drive which more than often gets him into trouble. For this reason, Susan Vandom has separated from him before the start of the series, but Thomas was deceitful enough not to agree to a divorce since he counted on her (most unwilling) support in case he needed more money for his dubious dealings.To this end, he gave her the choice of either providing money for him or lose guardianship over Will in a court trial, but with ex-Guardian Kadma's (financial) help the crisis was averted, and Vandom has since departed from his former family's life never to be seen since.
In the TV show, he is Tony Vandom, he is divorced from Susan Vandom, and is engaged to Serena. He appears in Season 2 of the animated series. Tony is rather wealthy owns a large aparment and has acesses to the building's swimming pool as well as a convertible. It is unknown what caused Tony and Susan to divorce in the animated series though judging by their fighting it is implied it came from Thomas's less responsible side. However, he is shown to have a far better relationship with his daughter Will than in the comics and actually cares for her. In "N is for Narcissist", he offered to let Will and her friends skip washing cars for a ride in his convertible. Much like Luba in the Television Series Tony is a much nicer person.