Which Buffy character was introduced as a Watcher sent to replace Giles in season 3?

The Buffy character introduced as a Watcher sent to replace Giles in season 3 is Wesley Wyndam-Pryce.

Wesley first appears in the season-three episode “Bad Girls,” portrayed by Alexis Denisof. The Watchers’ Council assigns him to oversee Buffy Summers and Faith Lehane, making him a replacement figure for Rupert Giles while Giles remains involved in Buffy’s life. Wesley is younger, highly formal, and much less experienced in practical fieldwork than Giles.

His early characterization is deliberately comic: he is pompous, rule-bound, and frequently ignored by both Slayers. The name can cause confusion because Wesley’s father is named Roger Wyndam-Pryce, while the character’s surname is also spelled “Wyndam-Price” or “Wyndham-Price” in some references. Those variants refer to the same character.

Wesley eventually becomes far more significant in the Buffy universe. After being dismissed from the Watchers’ Council, he joins Angel Investigations in the spin-off series Angel, where he develops from an ineffectual Watcher into a skilled and morally complicated demon hunter.