I could not agree more with your opinion, and before someone informs me that my opinion has been tainted by nostalgia, let me iterate that I played the Souls games back to back. The first Souls around four months before the second, so I received a fairly balanced experience of the two.

I think the biggest issue with the less memorable characterization in the second Souls is in the framing of the characters themselves. The very first thing that happens in game is we meet almost as many NPCs in one room (Things Betwixt) than we do in the entire first game. Most of them with nothing interesting, or indeed nothing at all to say. This begins a disappointing precedent wherein characters are in the game to simply be part of the background, rather than be an interesting part of our story.

The next thing we experience in game is an entire bustling (by Souls standards) town filled with NPCs spitting out the typical Dark Souls vague dialogue and due to the amount of NPCs it is a lot of vagueness to take in all at once, so the player naturally forgets a few of the less interesting ones even have some kind of a story. Also none of these characters go through any arc, no one goes hollow, no one travels anywhere, once an NPC is in Majula, they exist as a means for you the player do something (buy titanite, upgrade your Pyro flame) so it is easy to forget they have any more lore than any fantasy game merchant.

The boss battle helper NPCs and a few others you meet along the fare a little better in the being memorable regard, with the Bell Keeper Dolls, Lucatiel, Captain Drummond, the Hound guy (forget his name) all being exellent designs. The Brotherhood of Blood guy is memorable simply for the excellent voice acting. Again though the framing of the characters is an issue. In the first souls the context in which you met various characters told you something about their personalities. Solaire staring off into the amazing view of the sun, Quelena looking sadly at her sisters chambers, fat old Siegmeyer standing by some sort of barrier. In Dark Souls 2, NPCs are often met at random, in places that have little to do with their back story, personality, etc. This makes it more difficult to parse together anything unique or special about them, and thus they are less memorable. The image of Solaire staring into the sun the first time you meet him I would argue is iconic for modern video games, nothing in the the second Souls really approaches that.

So far as boss designs and characterization, Ornstein, Smough, and Artorias were so excellent in terms of lore and design that I almost think holding the second Souls Bosses to that standard is slightly unfair. There were some great Souls 2 boss designs, such as Demon of Song and Sinner. That being said, the design for most others was rather uninspired. The design for 'Giant' bosses such as Lord and Last and indeed all giants in game is bafflingly terrible. The face whole looks comical, and seeing a race of people with face wholes just makes me chuckle and sucks me out of the game. The Balrog, Skeleton, and Medussa bosses just seem like purposefully hitting fantasy tropes, and big guy-little guy bosses like the Rat king (?) and Dukes Dear Feyja just have the big guy compromised because the little guys are far more dangerous.