Karl Haushofer カール・ハウスホッフ / カール・ハウスホッファ
The two Men in Black watch Lain from lamp posts and the Iwakura gate, wearing odd goggle-like eyepieces. Karl is a tall blond foreigner who smokes; Lin is shorter and more impatient. They work for an executive at Tachibana General Laboratories and are tasked with hunting down the Knights, but they are only partly told the plan. They tell Mika she never saw them, flee when an angry Lain damages Lin's eyepiece, and warn Lain that the Knights planted a bomb in her cooling system. Karl tells Lain that she herself is what is dangerous, and in Japanese notes he admits he feels something like love toward her without knowing what she is. Joji Nakata voices Karl and Takumi Yamazaki voices Lin; ANN lists the English actors under the pseudonyms James Lyon and Robert Wicks.
More (spoilers)
After Lain leaks the Knights' membership list, the pair assassinate members such as Masuoka. In Layer 12 their boss reveals he has sided with Eiri and tells them to flee somewhere without wires or satellite signals. A ghostly figure then appears; Lin dies in terror and Karl is left broken. Konaka says the ghost was shot on 8mm with Ueda and Nakahara under a blanket. After the reset they are construction workers.
Sources: en.wikipedia.org archived · ja.wikipedia.org archived · animenewsnetwork.com archived · konaka.com archived · 4gamer.net · lain.wiki