Who is the blonde actress in Dr.. who?

It's impossible to tell you definitively who the blonde actress in Doctor Who is without more context. There have been many blonde actresses in the show over the years, both as companions and in other roles.

To give you a more helpful answer, please tell me:

* Which Doctor? (e.g., Tom Baker, David Tennant, Jodie Whittaker)

* Which era? (e.g., Classic Who, New Who, specific season)

* What role did she play? (e.g., companion, villain, guest star)

* Do you remember any other details about her character? (e.g., name, personality)

Once you provide more information, I can help you figure out who you're thinking of.