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.
Kevlar is often paired with another string, like nylon, to blend the strengths of both materials.  This combination addresses the stiffness of Kevlar alone, which can be uncomfortable for tennis players, especially when used with a rigid racquet. Some suggest using a very thin Kevlar for increased comfort, but even in its thinnest form, it remains stiff.