Goblin slayer: Year one is a side story to Goblin slayer.
This series takes about 5 years before the original Goblin slayer. I am guessing that because the guild girl from the original story says she has known Goblin slayer for that long. The guild girl does still show up in this side story as a newbie guild rep.
We have a young Goblin slayer who just starts to learn his trade with costly trial and error. And we have a young warrior who is going through a tough time facing the reality of being an adventurer.
That is right. The story has two protagonists and two separate stories. While the two boys encounter each other somewhat frequently, each guy’s plot doesn’t really have anything to do with the other.
For Goblin slayer, he is a young lad in volume 1 who ventures into the town in order to register at the guild to hunt down only goblins. He does come across a younger version of the cow girl (Still huge rack, loli) who is his childhood friend. From get-go, the cow girl falls for him pretty much instantly although she is just too shy to make any moves, and Goblin slayer couldn’t care less. We also learn why he loathes goblins from the first few pages of volume 1. Without spoiling too much, his deep hatred toward them is perfectly justified.
As for the young adventurer, he forms a party of four and meets a young, charming, archer. For a little while, he remains very happy and even thinks about marrying the girl. However, the reality soon strikes. They counter a very tough rock digger (enormous mealwoorm-like creature) that swallows the girl, killing her pretty much instantly.
The rest of the young warrior’s story is about redeeming himself and getting over the girl’s death. He will partake in the hunt for the monster without a much care in his own life. He becomes basically suicidal.
For Goblin slayer, his story is about learning goblin’s tactics and their weaknesses with trial and error. Without spoiling too much, at first all he has is just sheer tenacity. To his credit though, he learns really quickly.
We still don’t get to see Goblin slayer’s face. But we do get to see a younger version of the cow girl. The guild girl also appears as a newbie in this series but she hasn’t changed a bit.
The cow girl, though, has a long hair in this story, initially at least.
Overall, I don’t find this side story as interesting as the main story. Things just aren’t as interesting because there isn’t much going on with Goblin slayer himself, and the younger warrior’s plot is a little too predictable. Even his personality is cliche; he is a typical hero type.
Goblin slayer himself is a rather useless person to have a story with, so he needs interesting companions. In the main story, he does have such companions. In this story though, he acts mostly alone. This is probably why the young warrior arc exists. It would be awfully boring to just cover Goblin slayer who speaks very little words and just goes on about massacring goblins.
It does have a lot more fan service scenes if that is your thing. Most of such scenes come from the cow girl who is too low in self-confidence to do anything about her childhood friend, Goblin slayer. She does grow up mentally slightly in later volumes but not by much.
So, my final verdict on this manga is: Pass it. You are not going to miss anything as long as you’ve read the main story.