"Niko is Link's roommate in Spirit Tracks. They live together in a house along the beaches of Aboda Village. Similarly to Phantom Hourglass, he will tell Link the intro to the game, which is a summary of the Spirits of Good's fight against Malladus, the Demon King. This is the same Niko from the previous two games, being evidented by him saying that Link in his Recruit Uniform looks just like an old friend of his when he was young,[1] and by when he gives Link the Shield of Antiquity, he says it belonged to an old friend of his from his adventuring days,[2] and considering that this shield is the same one Link has in Phantom Hourglass, it is a solid connection to the previous game."

www.zeldadungeon.net/wiki/Niko

For the Link thing, it has been said that Link is merely a spirit. A heroic one that goes from body to body, era to era, to find the hero they need. It has nothing to do with blood. Zelda is the opposite, as it's always her blood that causes Hylia's powers to activate.

As for the Hero of Winds, it was said in Spirit Tracks by old man Niko. He claimed he was Link's roommate for a long time, while Tetra went off to create the new Hyrule Kingdom. It's possible they merely lived together because neither could stand living in the castle, but it's more likely that Tetra and her following family had no reason to keep them around. Tetra had a husband, and no one really liked Niko too much in the pirate crew.

"Niko is Link's roommate in Spirit Tracks. They live together in a house along the beaches of Aboda Village. Similarly to Phantom Hourglass, he will tell Link the intro to the game, which is a summary of the Spirits of Good's fight against Malladus, the Demon King. This is the same Niko from the previous two games, being evidented by him saying that Link in his Recruit Uniform looks just like an old friend of his when he was young,[1] and by when he gives Link the Shield of Antiquity, he says it belonged to an old friend of his from his adventuring days,[2] and considering that this shield is the same one Link has in Phantom Hourglass, it is a solid connection to the previous game."

www.zeldadungeon.net/wiki/Niko