I may be biased because I use these characters, but other than obvious other top tiers, I'd say Samus or Metaknight, depending.
Samus because she's absolutely a technically-oriented character, possibly moreso than Lucas. What Samus does is contradict Lucas in almost every other way. She's more about controlling the field and getting solid punishes, where Lucas is more of seek and destroy, so to speak. Also, Samus doesn't get comboed the same ways Lucas does, so if you have a good Samus to backup your Lucas, it could really throw some opponents off. Plus, not many people play Samuses, making her harder to have solid information on. But most importantly, from what I know at least, she's known to upset some MUs that I think give Lucas a harder time like Link, Falco, maybe Sonic? I'm not too sure, but generally she seems to me like a better idea than most characters.
For similar reasons, Link/Tink may work. Link if you're capable of using Projectiles for control, Tink if you want to play more like Lucas than Link but still be different to an extent? I don't know.
Metaknight I say because he's a lot like Lucas. Very much about rushing-down your opponent, and in theory he's like Lucas in the getting comboed and dying early department (albeit as of 3.02 Lucas doesn't have this weakness, but he will in 3.5) for different reasons than Lucas. Metaknight is just plain easy to kill off the blastzones, but has a really good recovery. Lucas in 3.5 will have the better survivability weight, but will have a worse recovery. Otherwise, they're a lot alike. Multihit aerials for pressuring/fear tactics, really solid kill moves, amazing movement options that ultimately serve the same purpose. To me, Metaknight's multijump baiting/mindgames/mixups are just as potent as Lucas' crazy horizontal movement, DJC, and magnet cancelling options. And, again, they work to the same ends.
As I said, this may just be my experience, but I felt that Metaknight was the easiest to pick up. But then again, I don't know how my playstyle differs from yours, so...
If you're just looking for matchup coverage, look for a character who's as opposite to Lucas as possible without being too low-tier. Heavyweights and projectile-heavy characters come to mind. However, keep in mind that some characters like Diddy, Lucas himself, etc, are likely getting the nerf hammer very soon.
Other than that, my only advice is to think more on your own terms of what a secondary should be. What/who feels most "right" to you?