Being an Angel has its perks...For one thing, they know their purpose. Take Kowaru, for example. Throughout the establishment of his character's persona, we discover that he is very calm and knowing. He seems to be very serene and sees the whole picture. When he discovers Lilith behind "Heaven's Door," he does not approach her because he knows that reaching her means the end of mankind. Instead, he sacrifices himself. This makes Kowaru a very noble and selfless character.
Kowaru is also very intelligent. In the short time he is in the series, he informs Shinji of many things concerning the human race. He explains the route of unhappiness, the frailty of the heart, and how all men are "fundamentally alone." He also makes Shinji aware of his fear of getting hurt and betrayed. He makes Shinji feel understood.
Nagisa Kowaru can be described as mysterious. Before it is known that he is the 17th Angel, he has everyone at NERV guessing as to who he is and why he is so...strange. His appearance is similar to Ayanami Rei's, who we know is not very human (the only way I can put it without it being an actual spoiler...). What really intrigued me about Kowaru and made him my favorite male character was his appearance, plus his appreciation of fine music. Angel or not, Kowaru is definitely my favorite male Eva Pilot.