Although he's one of the first heroic spirits encountered, his involvment in the war is limited. He's a free spirit and enjoys the grail war mostly for the battles. His actions are often cowardly, but counter to this he shows a deep sense of honor. This causes conflict between him and his master, but as a servent he is compelled to obey.
As class lancer, his spear gives him a medium fighting distance not well suited to being too close or at range. This means he prefers to be on the offensive. While not oriented towards defense, he is said to have the highest agility of any servant, and is quite effective at avoiding attacks.
His noble phantasm is Gae Bulg, which attacks a single target. While not exactly spectacular, Saber relates to Shiro that it's perfectly suited to killing a single servant in a contest such as the grail war. Gae Bulg is an attack which ALWAYS pierces the heart. It cannot be avoided. In fact it can kill even if the trajectory of the spear is not aimed at the target, or at the correct range.
In his past life he was Cu Chulainn: a legend of Irish history. This comes in to play not just in his lust for battle, but also in his whimsical attitude and slightly old fashioned thinking. While not focused on much, he's also a ladies man.