
There's three ways to play Daisy Walker. Okay, possibly more, but these are the most obvious. :)
1) Detective Daisy! You can make her into Nancy Drew extraordinaire; backed up by a large amount of Seeker cards, Old Book of Lore, Dr. Milan Christopher and her amazing 5 Knowledge. In a campaign she'll probably eventually complete her Higher Education and figure out the Sordid Secret of the Strange Solution, giving her both great Willpower and some decent fighting skills.
PRO +The advantage of this build is that Daisy will be a great clue gatherer with access to both card drawing, extra resources and even a bit of combat. +It's a versatile and fun playstyle. Seekers tend to be a valuable party members and you can often help your friends (free cards! Medical assistance!) if you have spare time.
CON -Rex Murphy is arguably the better clue gatherer, while not being as versatile as Daisy. I personally think Daisy is more fun to play, but that's like, just my opinion, man. -Detectice Daisy doesn't have room for more than a few Mystic Cards. Ward of Protection, maybe even a Shrivelling and a Holy Rosary, but she can't really go Team Purple when so heavily invested in Team Brown. -Even with a few combat cards you'll never be a scrapper. Bring someone to babysit you and don't split the party (too much).
2) Witchy Daisy! You can play Daisy dual-class: part-time sorcereress, part-time detective. This will give her decent combat abilities in form of cards such as Shrivelling, access to plenty of powerful Mystic cards and still let her perform well in her role as a clue gatherer. You WILL need to add several support cards such as Holy Rosary and Higher Education to make up for her 3 Willpower, but it's doable.
PRO +So many options! You can Scry, you can Shrivel, blind your enemies and unravel the mysteries of the universe. And you can still investige better than most.
CON -I feel that the Mystic cards, while powerful and versatile, always have a high cost. They demand a premium of resources, actions and/or sanity. It's easier to cope with this if you specialize and Daisy will never be able to do that as well as her as her more arcane colleagues.
3) Helpful Daisy! Daisy is great at support - her Seeker/Mystic combination and free tome action ability is a really powerful combination. She can heal damage with Medical Texts and Horror with Clarity of Mind. She can let her friends draw cards with Old Book of Lore, she can Scry, she can use Ward of Protection. And if that's not needed, she can still investigate better than most.
PRO +No one can support as Daisy (at least not 'till The Path to Carcosa is released).
CON -The more support cards you'll include, the less useful Daisy will be on her own. -This game isn't really dependant on support characters. Time is of the essence, so why help other people do stuff when you can just do it yourself? You could argue that support-Daisy allows the other investigators to specialize more, but she's still spend precious actions on healing when she could be advancing the story.
In conclusion, I prefer option #1, as I think playing on Daisy's strength as a Seeker is the better choice. There's still room to dabble a bit in mysticism and support cards, so you have lots of variety. Options #2 is a completely acceptable way of playing Daisy as well; Witchy Daisy can be super-versatile, especially if she's the only spellcaster in the gang. PS: I would think twice before playing her in a Hard campaign - her 3 Willpower will cause problems.
Option #3 is a bit more complicated, as it's (obviously) multiplayer only. You'll need to design your deck to fit in with the group and your group need to design decks that actually need your support. It's a great option but for advanced players only.
If you're like me and tend to get bored with one-dimensional characters (looking at you, Zoey!), you'll love Daisy. Yeah, she's a seeker at heart, but access to a huge card pool and a free tome action each round gives her more options than most other investigators. And options = fun. :)