Sentaro Kawanishi (川西 仙太郎Kawanishi Sentarō?) is a boy who loves riding his bike around the neighborhood in Nekomi. He meets Skuld during the time where she was learning to ride a bike. He was the one who taught her that it did not matter if she fell off the first few times she tried, what mattered was that she got back up, and eventually she learned. This coincided with Skuld's magical powers slowly developing—it was revealed that a Goddess' power comes from loving others. Skuld started to fall in love with Sentaro, and hence her powers started to develop. Sentaro eventually moves away with his family, though Skuld does not become sad by it, as she claims that despite the distance they will always meet when they want. It turns out that Sentaro only moved to the town just across the river. Urd's comments regarding this embarrassed Sentaro and angered Skuld. It is unclear whether or not Sentaro is aware that Skuld is a goddess, but he has seen Skuld use her powers, has heard her state that she is the goddess of the future, has seen Skuld's angel, and he has stated (albeit through inner-dialogue) that she looks like a goddess.
Sentaro is the teenage boy that Skuld meets in the Childhood's End story arc of the manga. Although they don't have a "real relationship", it's clear that she's somewhat attracted to him, thus signifying "Childhood's End".
It's clear that Sentaro's role in the story is to help develop Skuld. Skuld first meets Sentaro when she quits attempting to learn how to ride a bicycle whilst running away from home (because she felt that Belldandy didn't care about her), and when she runs away she meets him. After talking a bit, Sentaro teaches Skuld how to ride a bike. That is the beginning of their relationship.
Later, Skuld begins to realize that she has feelings for Sentaro, especially since Belldandy states that a goddesses' power comes from her love for others; since Skuld is learning how to love, her powers are beginning to manifest.
Although slightly sweet, Sentaro has yet to appear outside of these few stories (since we only get to see him in a few arcs). However, in the Gate arc, Skuld (after being kissed by the ever-curious Gate) mentions that she hasn't even kissed Sentaro yet, indicating that their relationship is still at kindergarten level, much like Keiichi and Belldandy.
Sentaro has also been part of another arc which begins in "For your sake I'll fly-y-y" (ch.216), were we can see some progress in his relationship with Skuld (which now is a little more serious). There we can appreciate that they express how much they care for each other with some small words (Skuld admitted that he liked him). Also, it hasn't been confirmed that they have a "real relationship" as said before, but Urd having the thought of Sentaro being unfaithful to Skuld gives the idea to the spectators that they actually are that close.
Apart from that, they are mostly compared to Keiichi and Belldandy not only because of the "kindergarten level relationship", but because the most important thing for them is being together, leaving behind the physical contact (still, Belldandy tried to convice Sentaro to kiss Skuld, which he didn't do). We can see that in this same arc, Sentaro was worried about moving away, since he wouldn't meet up with Skuld much often, but she told him that they could go anywhere side by side (also that she could go to the other side of the Earth to be with him). That is somehow similar to Belldandy and Keiichi constantly repeating that they will be together forever, no matter what.