Mikayla is a prostitute that Leonard, Raj, and Howard met during a trip to Vegas.
Mikayla approached Raj and flirted with him. Raj responded favorably until Leonard pointed out what her line of work was. Shortly thereafter, the two used their casino money winnings to pay her to entertain Howard, hoping to cheer him up after his recent break-up with Leslie Winkle. Mikayla offered to give their friend the "girlfriend experience" (which involved not revealing to Howard that she was a prostitute), to which Raj suggested making it the "Jewish girlfriend experience".
Howard quickly saw through the ruse, but was nonetheless grateful to his friends, particularly since she'd already been paid. ("The Vegas Renormalization")
Howard's encounter with Mikayla would get him into trouble some years later when Bernadette saw a YouTube video of Raj recalling that night at Howard's bachelor party. ("The Stag Convergence")
Mikayla was portrayed by Jodi Lyn O'Keefe.
Raj and Leonard approach Mikayla.