Mishti Patel is an old childhood friend of Baljeet's from India who pays him a visit when Phineas and Ferb built an ocean liner ("That Sinking Feeling"). There's a possibility that they like each other. Baljeet is apparently shocked at how much her appearance and apparel has changed.
Nothing is known about her early childhood except that she lived in India and played soccer with Baljeet, where she became good friends with him. Mishti apparently stayed behind in India when Baljeet moved to America. She seems to have a tomboyish nature.
She might be a love interest to him but nothing is currently known about their relationship except that Mishti is good friends with him. While they played near the end of 'That Sinking Feeling' like friends they were seen later holding each other by the sunset, which was apparently enough to convince Candace that Phineas and Ferb had 'created romance' between them by arranging a romantic meal for two. Although at the end of the episode, she says "I just want to hang out like we used to."
Mishti is very mature for her age. She has dark brown hair in a bun with long strands of side-bangs and an inverse widows' peak at her hairline (At the flashback, she is wearing pigtails instead). Her clothes (as seen in her current age) consists of a rose colored top, a pink skirt and pink sandals. When she boarded Phineas and Ferb's Love Cruise, she changed into to a pink dress with a purple belt. Her eyes are almond shaped and brown with long eyelashes, which means she might be wearing mascara. Since she is shown wearing lipstick and/or lip gloss, it could be concluded she wears at least light make-up.
Her first name is Hindi for "sweet", and her last name, Patel, is very common in India, and historically meant "headman" or "chief".