What are you supposed to do with the Giant Ant in the Gutter

While nobody seems to have an authoritative answer yet, here's my current understanding, based on what others in the Souls community have managed to suss out:

• Agility seems to increase the speed of your roll slightly. It's also hypothesized to lengthen your roll's invincibility frames, but this can proven extremely difficult to confirm/disprove (even EpicNameBro, the author of the official strategy guide, was apparently unable to get a straight answer out of From Software on that topic). It theoretically also increases the speed of other actions such as how long it takes to drink from your Estus Flask. We know that it most likely doesn't affect spell casting speed or weapon attack speed. Additionally, this side-by-side comparison video (85 Agility vs. 95 Agility) seems to show essentially no noticeable effect on shield-readying speed.

• You get maximum Agility returns from Adaptability up until ~38 ADP. This breakpoint may change slightly depending on your Attunement stat, which also has a very minor effect on your Agility stat.

My personal (educated) conjecture is that Agility is no doubt useful on paper, at least based on what we think it currently does. Even in that case, if you're really good about timing your rolls perfectly, it's possible that you may not particularly need the lengthened invincibility window. The various other effects may or may not be useful to your character, depending on your build and playstyle. For example, a character that focuses on staying at maximum range may not always need the quicker Estus Flasks or roll speed, but heavy melee characters might want all the help they can get in those regards.

In any case, if you're planning on doing a lot of PvP or you just want to be a little bit quicker overall, I'd personally suggest spending a few points into bumping up your Agility a bit (most typically through Adaptability). However, if you're getting along in the game just fine as it is, then you may not need to worry about it.

I haven't discovered every last secret in the game, so I can't claim to be authoritative, but based on what I've experienced in this game so far and from my experience with prior games in the series:

• You can't participate in any kind of co-op or PvP, obviously.
• You can't read or write messages.
• You can't meaningfully participate in (or earn rewards from) any convenants that require PvP or co-op interaction to do so.
• Certain boss fights or protected areas that are designed to bring in other human players as opponents will probably use AI-controlled NPCs in those players' place.
• All of the story/NPC elements are almost certainly still accessible, as NPC progress is solely based on your solo exploration and interactions, which are mechanically unchanged in offline mode. This is also how it worked in both previous Souls games.
  • The sole exception to this may be any covenant NPC dialog or events that are only accessible after you've ranked up in one of the aforementioned co-op or PvP-oriented covenants.