There is a free hand highlighter tool, but it makes highlighting multiple lines of text tedious because it requires dragging across each line of text in a paragraph.

However it is currently possible to press and select text and then tap on the "highlight" option in the menu that appears. However, this requires using a finger and is not possible with a stylus. It also requires multiple steps to accomplish.

A better solution (and one that other apps have) is to have a separate tool that the user can use to only highlight text. Ideally, the user would be able to select the desired color and erase any highlights.

This would be an incredible and (theoretically, because the functionality already exists in the app) simple tool to add and would make annotating textbooks and articles much easier.