During Declan's first appearance, he passed himself off as a criminal profiler when "competing" against Shawn Spencer to track down a murderer. But later, he revealed that he was not a criminal profiler, but that he was rich and with a lot of time on his hands, which he decided he wanted to use to solve crimes.
He came clean about himself to Juliet O'Hara, whom he began to date not long afterwards. Juliet's relationship with him, however, became complicated after she overheard Shawn say that he had feelings for her.
Even after Juliet kissed Shawn (in Declan's house), she remained with Declan for a very short while, but broke their relationship off after traveling with Lassiter to Vancouver (instead of going with Declan on the two-week vacation he had planned for the two of them) and finding herself with the possibility of a 'moment' with Shawn (which, after a failed attempt, did occur).