He played college football for the Northwestern Wildcats before being selected by the Chargers in the first round of the 2005 NFL draft.

He claimed he used the steroids in an attempt to fully recover from an elbow injury suffered in the very first game of his senior year at Northwestern.

Castillo hyper-extended his elbow, damaging the ulnar collateral ligament, basically preventing him from using one of his arms.

"[6]After the urine test came back positive, he and his agent wrote the letters to all the teams admitting use of an illegal substance.

Despite this, San Diego Chargers Executive Vice President and General Manager A. J. Smith took a chance on Castillo because of his stellar track record at Northwestern.

Castillo has emerged as a play maker alongside Jamal Williams and Igor Olshansky, creating havoc in opposing backfields.

In the 2006 opening game at the Oakland Raiders ESPN commentator Dick Vermeil called Castillo one of the best young defensive linemen he's seen in a long time.

His season totals included playing in 10 games (9 starts) 37 tackles, 7 sacks, and an interception.

[8][9] Castillo started in all three playoff appearances for the Chargers, including the AFC Championship against the New England Patriots where he sacked Tom Brady.

His statistics for the season were 16 games played, 15 starts, 39 tackles, 1½ sacks, a pass defensed and an interception (the second of his career).

He was born in Brooklyn, New York and moved to the Dominican Republic with his mother, Maria, when he was a child.

During his trips to the Dominican Republic, Castillo hosts a youth football clinic and does many community appearances.

Each child who participated in the event enjoyed dinner with Luis and his teammates and received a holiday gift card from Walmart.