One thing that I find both fascinating and deeply sad in TSatS is that when Will asks if they're going to Percy's house to ask him to come with them to Tartarus Nico says no on the grounds that he doesn't want to involve Percy in it... but when asked why Bob wouldn't ask Percy for help, Nico's immediate thought is that Bob didn't think Percy and Annabeth would help him. Like, that's his one and only theory. I suspect that there's a part of that where... Nico thinks to himself that Bob doesn't think Percy and Annabeth would help so that he doesn't have to think that he doesn't think Percy and Annabeth would help. In other words he's thinking it's Bob who didn't call for Percy and Annabeth because he didn't believe they'd help him, because the alternative is for Nico to say "I'm not asking them for help because I know they would say no and I don't want to face that refusal". And it's not that he wants them to come! If Nico had had his way he would've gone alone, he doesn't want to put anyone in danger, it's just that he managed to find the only demigod as stubborn as he is and made the mistake of telling Will what his plans were. But it's one thing to not want to involve people in something incredibly dangerous; it's quite another to know that if you asked for their help, and in this case their help saving the person they owe their lives to, they would say no. I'm sure they would be properly apologetic about refusing, they do seem to feel genuinely terrible about forgetting Bob (which... good) and "I'm not going to superhell again" is a perfectly understandable boundary to have, but I think Nico just... doesn't want to admit to himself that he was willing to go to Tartarus for Percy but Percy would never do the same for him. Hence him leaving it at "I don't want to make him do this" when asked if he'll ask Percy to help for his sake, but thinking quite openly to himself that Bob probably didn't think Percy and Annabeth would help, because Bob helped them out of loyalty to Nico so it isn't devastatingly sad to admit they'd never do the same for him the way it would be for Nico to admit that Percy would never go to Tartarus for him despite him going to Tartarus largely to help Percy.

Also, it's deeply disappointing that it doesn't come up again later in the book because please for the love of god Rick can we please have a discussion around Nico's fatal flaw being his "will literally go to hell and back for people who he knows would never do the same for him" level of loyalty and not holding grudges, and also how literally the only evidence even his own sister could offer for holding grudges being his fatal flaw was that he's the son of Hades and also was still upset about his sister dying six months after it happened. Now that he's got someone as loyal to him as he is to everyone else (Will refusing to not follow Nico to hell is so good after so long of Nico constantly being prepared to give up everything for others and getting next to nothing in return) it's a great time to get into how despite basically every other POV character going on about Nico being creepy and morally dubious actually his most consistent character trait is being the most loyal character in the whole series (and possibly the whole Riordanverse, honestly) no matter how many times that loyalty burns him.