These two do indeed appear in the main series, and bring the awesome.

At some point I will switch Heinkel’s hair color from blonde to light brown, because there are Too Many Blondes In Hellsing. (Count ’em sometime.)

And now it’s time to pay a visit to the doujinshika’s inbox

Erin: Ooh, we’ve got a double-header here… Well, sure, I can answer two questions at once.

Email: One: How come you spell Integra’s name with an “L” at the end but also the standard way w/o the “L”, and Two: when are Yumiko/Yumie and Heinkel introduced in the manga?

Erin: Integra’s full name in the manga is given as this thing:
サー·インテグラル·ファルブルケ·ウィンゲーツ·ヘルシング.
That’s “Sir Integral Fairbrook Wingates Hellsing.”

or it could be something like “Farbrook”, but the accepted romanization is “Fairbrook”.

In the anime they drop the “Fairbrook”, as you can see on this scan from the CD.

And in both the anime and manga it gets shortened to just “Sir Integra.” (Gee, I wonder why.) So “Integra” is just a nickname for “Integral”, like “Alex” for “Alexander.” And both spellings are equally correct.

Speaking of Alexander… these two are co-workers of his. The blonde Austrian is Heinkel and the Japanese girl with glasses is Yumiko. (Without her glasses, she’d be Yumie.)

If you haven’t met this pair, get ahold of either of the first two manga volumes! In the back of each is a Cross Fire story— one of a few one-shot pieces that Kohta Hiraino did before starting Hellsing proper, featuring these two Vatican assassins. I’m also told that they show up in the main text of the sixth volume… yet another reason to want it out, and soon!