If you want this ASAP finish the third challenge in the first group, "Chuckie at the 'Lympics", to unlock Phil & Lil. You win just by getting into 1st place within 30 seconds so use the start boost, and there are 2 chances to grab items so use them.
If you are doing the races first you will unlock this later. Because no Chiefs are awarded on Slow, the first/easiest Chief to unlock is Winslow. You receive him for winning the first cup (Hairball Cup) on Medium difficulty. I finished first overall but with 2/3 stars. You just have to reach the top 3 in order to unlock the Chief.