An Undying creature will always come back from the graveyard when it dies, provided it doesnt have a +1/+1 counter on it
From what we can tell, these will work similarly to how past Japanese and Phyrexian cards have worked, meaning there will be card styles you'll need to collect
It is then returned to the battlefield in a condition similar to what it was when it was first cast triggering any enters-the-battlefield triggers again and gaining summoning sickness if it was a creature
Precons are easier for beginners, but brewing teaches you more about Magic
there are a number of articles out there about this topic, with a variety of answers to go with all those articles
I find it hard to review her without the context of The Spear of Leonidas as well, and I think as much as it feels like the Spear should have auto-equipped, I think itd be tough to justify her at even four mana for that much value