Simon and Theodore.
Alvin is charming, carefree, musical, adventurous and fun-loving. He always seems to come up with hare-brained schemes to accomplish whatever his goal at the time is; whether it be trying to help his brothers, gaining fame or attention, becoming rich and popular or keeping Dave from figuring something out, or getting out of a sticky situation. However, Simon points out to Alvin that his crazy plans could get them in trouble and encourages him to do the right thing.
Alvin can sometimes be very mischievous, selfish, impulsive, egotistical, stubborn and greedy at times, he irritated or angered others around him, most obviously Dave. Other times Alvin's determination can be viewed as being thoughtless or manipulative. This can all be a constant habit as he has gotten his brothers or himself in trouble quite a few times. But despite all this Alvin does care a lot about his brothers and when he goes too far he apologizes and sets things right for his family, friends and loved ones.