Both Emma Frost and Kitty Pryde were introduced in X-Men #129 in 1980. In the 1990’s, Emma was developed into a “hero.” In 2004, 24 years after they met, Emma and Kitty found themselves on the same team.
(I hope this is not too small. I loaned my TPBs, Trade Paperbacks, out of a friend never to be seen again and it was either wait until tonight when I can fish this issue out of the longboxes or pull it from online.)
Astonishing X-Men #2, by Joss Whedon.