Kevlar is often strung with another string, such as nylon), in order to combine both strings' qualities, as Kevlar by itself feels too stiff for many tennis players, especially when combined with a stiff racquet. Some advocate using a very thin gauge Kevlar for increasing comfort, but even in the thinnest gauge it is a stiff string.
The use of Kevlar in tennis racquets, particularly when paired with nylon, addresses the issue of Kevlar's inherent stiffness.  While a thin gauge of Kevlar can be used for increased comfort, it still maintains its characteristic stiffness.