M. Bouc is a something of a big shot at the train company that runs the Orient Express, and he's an old friend of Poirot's. He is Poirot's companion and sidekick during the investigation, a type you'll often see in detective fiction – he's kind of like Sherlock Holmes' Watson. His rather slow wits are a contrast to Poirot's quick mind and penetrating insight, but they also give Poirot a reason to elaborate on his findings, which is pretty useful for the reader.
M. Bouc is often sidetracked by his own prejudices. That is, his thinking is driven by stereotypes, such as when he decides that the Italian man killed Ratchett, because "he is an Italian, and Italians use the knife! And they are great liars! I do not like Italians" (2.10.51). In this sense, M. Bouc offers a bit of comic relief (not to mention political incorrectness).