Literally a lifesaver for climbing a long length of vertical rope is a set of ascenders. This is a simple mechanical device that helps you get a good grip on a rope with your feet and hands so that you can climb straight up. Climbing a rope without a good grip or without any way to lock your position gets tiring very quickly. The ascenders, also known as "Jumars" (which is a brand name) are aluminum or composite metal loops with a rubberized handle and a bite clamp to hold on to the rope when weight is applied and to release when you want to raise your grip. Grip the top one with your hands and place one foot in the lower one and you can "caterpillar" your way up a rope. This simple tool makes the best support when you just need to hold onto the rope or in an emergency to climb.
With a set of ascenders made of a light alloy material it adds very little space and bulk to have them in your pack, so it's a very good idea to have them with you as you are rock climbing, or even on a high mountain expedition where you may run into trouble with ice. Falling into a crevasse where you have a main rope attached to your harness, you'll probably have too much weight in your pack, your hands will be too hold, and you may not get a good grip on the rope, so having the ascenders as a foothold will help you keep a grip. An alternative is to ascend with friction knots, which follow the same principle of gripping onto the rope when you place weight on the knots, and releasing when you pull up. For that you will need some strong lines, about 8mm in diameter, and a good knowledge of knots to make it work. You attach one knotted line, a prusik, around one of your feet and grip another one with your hand, and work your way up the rope. You will need to tie off the slack from the main line in a backup knot using a figure eight.