In Sims 3 Bella and Mortimer always start out as children as best friends. Left with no interference from me sometimes they get together and sometimes they don't. I became interested in Mortimer's welfare almost from the beginning. Then when I starting playing a female child as my main character I almost always tried to get her and Mortimer together. If he became partners with someone else though I backed off. In my games Mortimer and my sim would become partners more than any other. Then it would be Bella followed by Sandi French, Darlene Bunch and Kaylynn Langerak. Bella has had various partners also. In one game the Bachelors moved out of town.