Talk:Lei Cheung/@comment-2601:645:1:BCE0:DCE9:ECD2:7B26:F986-20190120230521/@comment-81.145.43.50-20190121163833

You need to rank up with 'The Dweller' to Grade 3 and 1/3 in order to learn about Lei Cheung.

I think it's the same for other Engineers you don't initially know about (ingame). Before you can even start unlocking them, you need to level up another Engineer, which will eventually 'introduce you' to another Engineer. The threshold is Grade 3 and ~33% of the completion bar to Grade 4.