Reminds of an older video where the guy was complaining that the tutorial pop-ups' font size is too big while the game was explaining stance switches, then went on to complain how the game doesn't tell you how to switch stances.
Anyway:
-AI companion: as the name says, AI. You don't switch to her, and since the game doesn't tell you how to switch to her, you obviously can't. It's not a BG style party based game, it should be obvious.
-Giving items: As kirottu said, press F, which brings up the inventory and at the bottom of the screen there's a big nice Enter button with Anjali written next to it.
-New abilities: Explained by a big tutorial text upon the first level up. Says that new abilities are automatically assigned to 1, 2, 3 the tutorial expects you to learn a new ability. Defensive skills are explained in the help topics (esc-> help topics) and are used with 4, 5, 6. Or while blocking, tho I can't remember where the game tell you that.
-Using abilities: Again, explained via tutorial text that you use them with 1, 2, 3. Being mapped to keys, it's obvious they're active skills.
-Talents: They're explained in the same text as above - passive improvements to your character.
-Life stealing: Esc->Help topics: "~ grants your attacks a moderate chance to drain life"
-Item use: If you would open your eyes, you would notice 2 things: Longsword is the item type, while the name of the sword, Rapier of Frost is shown above, with Lucas' mug behind it, suggesting the item can be used by Lucas. Other items don't have Lucas' face, suggesting they can't be used by him. Now, if you enter your inventory again and select Longswords, you will notice a Rapier of Frost in your inv, which you can lo and behold, use.
kirottu: the OP and he vid uploader is the same person.
Ah the wonders of not skipping tutorials, reading and opening your eyes..
NOW SOMEONE GIVE ME BACK THOSE 8 MINUTES AND MY DESTROYED BRAINCELLS