Cool Brian
Cool Brian, a Mentor For Young Boys was a program introduced at the beginning of the the ugly truth It was a company that sponsored somophores and college students to be mentors of young boys, like Rowley.
Rowley`s parents hired a Cool Brian employee for Rowley (who was rather old for it but due to his childish behavior he was very happy with it). Rowley was seen playing catch in his front yard with an employee. On his regular employee's day off, he played with a different "Brian", but, as Greg Heffley noted, Rowley didn't seem to notice.
Rowley's relationship with the company presumably came to an end after falsely believing he could "catch puberty" from the Cool Brians,because Rowley says he is scared of catching puberty.
Relationships
Rowley Jefferson
Brian is, as Greg puts it, sort of a "big brother for hire", who's job is to become a boy's "best friend," but Brian is mainly doing his job and does not really have strong feelings for Rowley, but Rowley doesn't seem to notice. At one point in time, an entirely different "Brian" was seen playing catch with Rowley, but Rowley didn't care.
Greg Heffley
Greg is actually quite jealous of Brian, because he was at first Rowley's "mentor" until Brian came along. But Greg's supposed "mentoring" was mainly teaching Rowley childish pranks, which is probably part of the reason Rowley's family hired "Cool" Brian. In Wimpy Boardwalk Island on Poptropica, Mr. Jefferson says that Rowley is hanging around "that Greg kid", implying that he does not like that. When you talk to Mrs. Jefferson, who is behind him, she says Mr. Jefferson thinks Greg is not a good influence, proving that it was probably the reason they hired Brian.