Clark in L&C does not have much in the way of ex-girlfriends.

However the Clark in Smallville has many. And many of them know of his powers. So take one of these and have her arrive in Metropolis needing Clark’s help.

Make her a sincere friendly ex with no current designs on Clark. No hidden get back together agenda,

It’s fine if Lois is concerned about possible hidden agendas.

I liked in Superman The Animated Series when Lana Lang showed up. She knew who he was because as she explained, I'm a fashion designer, changing clothing style is not going to fool me with someone I know.