Peter was an old suitor of Clarissa from the days when they met in Bourton. He has a lot of lines, and we can easily see his personality, and who is he. After they broke up, he moved to India, found another woman and separated with her, after that he falls in love again. When the novel starts we see him back to London and Clarissa. After that meeting we can easily see that it is not the end, they had something between them. Still, after so many years.
Peter reveals later to Sally Seton that Clarissa ruined his life by refusing to marry him. He rethinks much of their time at Bourton and decides to attend Clarissa's party even though he hates her parties. He waits for the entire party just to speak with her or be near her. To be honest, it seems to be so nice that a man spends half of his life loving only one woman. He wanted to see her even for one second, and it is completely excited to read for the reader of the 21st century.
Peter Walsh’s most consistent character trait is ambivalence: he is middle-aged and fears he has wasted his life, but sometimes he also feels he is not yet old. He cannot commit to an identity, or even to a romantic partner. He cannot decide what he feels and often tries to talk himself into feeling or not feeling certain things.
Peter Walsh in the Essays