Stacy is one of the major characters in Phineas and Ferb. She is the best friend of Candace. While reading through her wikia page, I found out that she is a Japanese American. Her nationality is only mentioned in the special episode "Summer belongs to you". Phineas and Ferb visit her cousins in Tokyo. Only mention about her native place and nationality is in the same episode when Stacy says
Say Hi to Jeremy for me and hey you should stop by and see my cousins in Tokyo.
When it comes to Baljeet, it is mentioned several times that he is an Indian. Similarly, is there a specific mention before this special episode that Stacy is of Japanese origin i.e., by some song or a dialogue?