Just asking to clarify.

Some people believe that page 23 of the rulebook defines a characteristic of bystanders when it says "Bystander tokens can be included in your force although they do not have keywords..."

Some people will say that that means that a character with a keyword could never be a bystander.

So, if I use "Welcome to the Legion" to give Lian Harper the Legion of Super-Heroes keyword, does she stop being a bystander?

no. She will be a Bystander with a keyword