I think they shared lead. Not sure who sang what, so it may have been Kimball he sounds like. By the way, well known fact: Eddie Van Halen played lead guitar on Michael Jackson's "Beat It." Little known fact: That's Steve Lukather on rhythm guitar.
Kimball was lead for that recording, Lukather didn't take over vocals until much later in their existence however I think he is the only member of the band from start to finish. I dated Bobby Kimball's niece when I was younger.
Do you remember who her Mom/Dad was? I ask because I was born & raised in the same town as Bobby & we went through 12 years of school together.