Kevlar is the brand name but the material is called Aramid. They are fibers that have more tensile strength than steel. Meaning they don’t break when they are stretched. It’s the same stuff they put into bulletproof vests.

It is the most durable synthetic string available,[12] and is thus extremely hard to break. However it’s lateral durability is not as strong, meaning you can eventually saw it in half. It is also the nature of a fiber. You can still cut it despite you inability to break it by pulling it from both ends.

Although it is one of the best strings in terms of tension holding capability (next to natural gut), it is the most dangerous string when it comes to developing tennis elbow. Kevlar is often strung with another string, such as nylon[10]), 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.

I personally will not even string Kelvar/Aramid with anything firmer than natural gut, PEEK or a Fluorocarbon. The risk of developing arm injuries is just too great.