Warning: this article contains spoilers for the fourth series of Sherlock

I’m not given to outbursts of brotherly compassion," warned Mycroft Holmes (Mark Gatiss) at the end of the third series of Sherlock. "You know what happened to the other one."

Over the last two episodes of the BBC One series, we've finally learnt who "the other one" was – Eurus (Sian Brooke), the distinctly dangerous sister of both Mycroft and Sherlock (Benedict Cumberbatch).

Here's everything you need to know about the character:

Her entrance had been planned for a long time

"The scene where Eurus finally unveils herself, we've been sitting on that since we sat talking about it on the production bus... when we were shooting His Last Vow," showrunner Steven Moffat recently revealed.

But the idea for the character began much earlier, as Gatiss – his co-writer – explains. "It started as a joke years ago there was going to be a line in the first season where we were sketching in the Holmes family," he told Radio Times. "But we thought we won't do too much, and thank God we didn't, because it has given us this place to go."

In another interview, Moffat revealed the joke that never made it to broadcast: "We made and, thank God, cut a reference in [2010] episode The Great Game... where Mycroft is explaining he is smarter than Sherlock, and then Mycroft goes onto say, ‘My sister of course...’ and then gets cut off.