Here is how to master the three distinct philosophies of underlining in Illustrator, and why you should probably never use the default "Underline" option again. Yes, it exists. Open your Character panel (Window > Type > Character) and click the "Underline" icon (the T with a line under it).
It draws a line exactly at the baseline of the font at the exact weight of the font’s stem. Why it fails: It doesn't scale well. It ignores ascenders and descenders. If you have a lowercase 'g' or 'y', the line will slice right through the tail. underline text in adobe illustrator
Use the for global underlines. Use Line Segments for artistic, variable underlines. Only use Outlines if you are physically warping the underline to wrap around a 3D object. Here is how to master the three distinct