I agree that this kitty is thin! And I can tell you with almost 100% certainty that this is a female, because almost all calico cats are female. Certainly she would benefit from some good nutrition in the form of a high quality food. I worry that the anus is so irritated because she has diarrhea, possibly from intestinal parasites, which could also be contributing to her failure to gain weight.
The red eyes could be a sign of an upper respiratory infection. Cats don't get what we think of as pink eye, but certainly they can have bacterial eye infections, and these need treatment with antibiotic eye medications. I would recommend you keep this kitty safe overnight in the house with some good food, and then take her to your nearest vet in the morning. He/she can scan her to see if she has a microchip, as she could be lost from her owners, and then provide appropriate medications.
If you need veterinary care in the future, please be aware that PetCoach offers housecalls to homes in the San Diego area. Check out:
https://www.petcoach.co/house-call-veterinarians/ or email us at housecalls@petcoach.co.