|Main Character: Miami|
|Birth Date||August 19, 1985|
|Rank||CSI Level 1|
|Portrayed By||Omar Benson Miller|
|First Appearance||Bolt Action|
He was originally part of the night shift of the Miami-Dade Crime Lab but transferred to Horatio Caine's day shift team in season 8. He is an art theft specialist who was promoted to the main cast after three episodes and is currently a CSI Level 1.
Despite being a "newbie" (even mentioning student loans at one point), Walter often has a good relationship with his co-workers in the lab, due to his friendly personality and sense of humor. He especially bonds with Jesse, who later passes away from halon gas. It is not known if Walter is, or has been, involved in any romantic relationships. He does, however, exchange jokes about that with his co-workers whenever they seem to be having dating flings.
Walter is generally a pretty laid back guy, and is normally cool and kind, but he stands up for himself, even to Horatio’s son, Kyle Harmon (episode 818, "Dishonor"). In such times, he will also occasionally have an intense, fiery, angry demeanor. Despite being a "big, strong guy," he will also get nervous at times when presented with uncomfortable situations.
Simmons appears to be a deep thinker, often "connecting the dots" between different pieces of evidence.
Despite his stature, Simmons seems to be easily frightened by large animals and human remains (episode 807, "Bone Voyage"). Hence he is rarely seen in the autopsy room and handles trace evidence instead.
As the lowest ranking CSI, Simmons is sometimes given the dirtier and unwanted tasks, such as retrieving a dead man flushed down a toilet and collecting trace from a large pile of rocks.
In the episode "Hunting Ground" it is revealed that he speaks French fluently while the episode, "Bad Seed", reveals that he also a strong knowledge of poisons and toxins which proves helpful to the team.
Much like Ryan Wolfe in his debut appearance, Simmons refers to Horatio as "sir" rather than "Lt" or even "H" which Ryan and Eric often call him although he has started referring to Horatio as "H" but still calls him "sir" at times.
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Simmons generally has a good working relationship with all the team members. His sense of humor has made him a likeable personality amongst his more senior colleagues. He was especially close to Jesse Cardoza and often played basketball with him. Ryan took him under his wing, and the duo have a friendly relationship, though Wolfe occasionally enjoys "pulling the seniority card" and leaves Simmons with the undesirable tasks.
Initially he was distrustful of veteran CSI Eric Delko when Delko was conducting undercover investigations in the episode "Time Bomb", but has since had a mutual respect for him after Delko exonerates Wolfe.
- Despite being right-handed, Simmons wears his watch on his right wrist.
- His father is a teacher.
- He uses a Glock 17 as his gun.