Omg, I'm a big fish nerd but I'm an even bigger Buffy nerd!
The Big Bad (major villain) of season 5 was Glorificus (Glory) a goddess who ruled a hell dimension with two other gods. Glory was the most powerful of the 3 gods and the other 2 worried that Glory would take over. Rather than let that happen, the 2 gods worked together to banish Glory from their hell dimension. She was put in our dimension (on Earth) and she was bound to a newborn male (Ben).
Glory and Ben have a weird relationship. When Glory saves up enough energy, she can take over the Ben/Glory body and when she does, she looks like a young attractive human female, and she has super strength (much stronger than Buffy's), super speed, and incredible endurance. However, when Glory uses up all of her energy reserves, she reverts to Ben, a normal human male med student. Ben and Glory know about each other, but have no recollection of what the other does when the other entity is in control. The only way to kill Glory is to kill Ben.
One thing about Glory: she wants to go home, no matter what the cost. The only way she can go home is by opening a portal, and in order to open a portal, she needs the Key, a swirling mass of cosmic energy. However, once the portal is opened, all the dimensions will bleed together creating horrible chaos for everyone everywhere.
Some monks decided that the Slayer was the best person to protect the Key. In order to make sure Buffy really made the Key a priority, they fashioned the Key in the form of a human girl, Dawn. Then they implanted memories of Dawn in everyone's mind so it would seem like she has always existed and nothing was different, and they also magically doctored photos and created paperwork for Dawn (Social Security Number, etc) in order to validate the memories.
Though everyone in Buffy's circle of friends (including Spike) knows that technically Dawn is only a year old, they still have memories of her and since she IS a real person NOW, they treat her like she's always been around.
Spike is also in love (infatuated) with Buffy, and perhaps feels that protecting Dawn will prove how loyal he is and will show Buffy how much he cares.
Now for the easy questions.
Xander has an episode where he's split into two people, each representing two parts of his personality-- a responsible, dependable, serious guy and a lovable goof. The responsible one decides it's time to move out of his parents' basement and puts down a security deposit on an apartment (he and Anya got engaged at the end of season 5, after all).
Also in season 5, the owner of Sunnydale's magic shop is killed. Giles decides it will be a good investment and purchases the store. He hires tactless Anya as a salesperson/aide/manager who seems to be his only employee. She actually does a good job of running the store and eventually takes over.
