Why do attacks keep inflicting 0 damage?
It means that you missed. Battle Goddess doesn't have a "miss" message when it happens, so the 0 is the only indication you have.
I just got my stats up to the point where I can survive this boss' attack, and suddenly he hits me for five times as much damage. What gives?
Likewise, the game has no "critical hit" message either. You got (un)lucky - better luck next time.
When I'm fighting with a companion, half our attacks don't do anything!
Battle Goddess uses the clunky old NES Final Fantasy targeting method. If two teammates are set to attack the same target, and the target dies after the first strike, the second attacker won't pick another target - she'll attack the empty space where the target used to be. You can easily counter this by attacking the enemies in the back row first, since the AI allies always attack the ones in front. It does make it take longer to autobattle through fights, though.
Why do enemies get to attack me twice in a row?
Magic in Battle Goddess is instant-cast, meaning that an enemy can attack you physically, then immediately unload on you with a spell. You can do this to them as well, but you'll need to learn to keep your health high to compensate for enemy attacks.
I'm pumping my STR/CON as high as it'll go, and enemies just aren't dying any faster/I'm not dying any slower!
One somewhat confusing thing about the BG stat system (especially for those accustomed to more traditional RPG stats) is that attack and defense power is not the key to being a better fighter - your attack and defense counters are. When an attack happens, the combatants' respective attack and defense counters are compared, and damage is assessed accordingly; having high power but a low counter is a bit like being able to bench-press 500 pounds but being too slow to get a good hit on anything. If a boss is wiping the floor with you, the stats you want to raise are DEX (for offense) and AGI (for defense). It takes a lot of points in each to see marked improvement, though; keep an eye on your AP and DP ratings to see when your abilities actually go up.
OK, I'm doing 999 damage with most of my attacks. Is there any point in boosting my attack any further?
Yes. 999 damage is merely the maximum that the computer will show you - hits can actually be much larger. This is why you'll sometimes see a ???-HP enemy go down after a single Super Neko Punch from Liliem, for example.
Will I see any familiar faces here?
Aside from Eushully-chan, Haishera, and Celica himself, fans attracted by the more recent Eushully games will probably not recognize any characters - although a number of them do return in Battle Goddess 2. However, the release of Battle Goddess VERITA includes Celica's early meetings with Revia and Suri, as well as a (very brief) appearance by Roca. Whether we will meet them again in the future remains to be seen.
How many endings does this game have?
Just one, although it is influenced by what you do in-game. The end credits are interspersed with additional (non-H) CGs of the various characters, assuming you've completed that character's sidequest (or main quest, for Roca and Revia). If that character has died or is no longer on speaking terms with Celica, you'll see a grayed-out image of their character model instead. The characters in question are: