[Assassin's Creed: Syndicate] Evie Frye
Here's more on Evie: http://www.gamesradar.com/assassins-creed-syndicate-female-assassin-Evie/
From this write-up, I would much rather play with Evie than Jacob, and it has nothing to do with their genders (honestly, I still don't care about that). Here's why (bold by me for emphasis):
The twin sister of Jacob Frye, Evie has her own story missions and skill tree and can be switched to at any time in London’s open world. She favours stealth over the instant violence of her brother and while we don’t know much about her weaponry, like Jacob she’s is a relentless Assassin and armed with the new rope launcher for hurtling up the sides of buildings like a Victorian Batgirl.
I'm getting tired of the hotheaded, "charismatic," kill-everyone-that-gets-in-my-way protagonists (Ezio, Edward, Arno) and would much prefer a cold, calculating character.
“Force has no place where there is need of skill." Herodotus
Wow, awesome! I would love to play as Evie as well, for the same reasons. Also we get two characters that we can choose from! Dude, this is amazing!
From what I've seen we only choose in open world settings. They both have their own missions to complete throughout the story, and probably need to be done in chronological order and not in a "100% Jacob, now 100% Evie" kind of way.
Edit to add: I wonder if this is a near compromise to Liberation's persona mechanic? Will they each have their own side content and collectibles that only one of them can do? For example, it would be much easier for Evie to play a prostitute to gather information from high ranking officials in a brothel then kill them than it would be for Jacob. 
I'm guessing that they will have their own side content and collectibles, and it would be a good thing. If nothing else, it would declutter the map if only half the content is available at any given time.
“Force has no place where there is need of skill." Herodotus
That's what I meant, and that's totally fine by me. I'd honestly almost prefer it that way, if it means a more tightly-designed experience. In the open world, I'll probably play as her a bunch more if the factoid about her having different Skills is genuine.
EDIT: [About Separate Collectibles and Side Quests]
THAT would actually be amazing, and they could do some great things with that. The only thing they'd need to add now is split-screen co-op so I can finally invite [REDACTED] over and she and I can wreak utter havoc on simulated London. She'd probably play Jacob and I'd play Evie though xD She's a brawler. I like sneaking.
EDIT: [About Separate Collectibles and Side Quests]
THAT would actually be amazing, and they could do some great things with that. The only thing they'd need to add now is split-screen co-op so I can finally invite [REDACTED] over and she and I can wreak utter havoc on simulated London. She'd probably play Jacob and I'd play Evie though xD She's a brawler. I like sneaking.
Evie gets the stealth content and political assassinations; Jacob gets the GTA carriage chases. There. It's settled. 
I don't see games without split-screen co-op adding it... I mean... there's a chance they'd lose out on a sale or two... 
“Force has no place where there is need of skill." Herodotus
A man can dream.
A man can wet dream.
FTFY.
“Force has no place where there is need of skill." Herodotus
I was thinking, how about a GTA V-type thing with 2 characters (Evie & Jacob) instead of 3 (Trevor, Michael & Franklin)? Each has their own missions, but they often work together on missions, so their paths cross very frequently. In free-roam, you can switch between them whenever you want and you'll fall into what the other was naturally doing while the player wasn't controlling them. The switching could be limited or forced in missions occasionally, when the situation calls for it.
Think about raiding a Templar hideout with your sibling, switching characters when it's advantageous (one can approach from the rooftops and scout enemies, while the other assaults from the ground under tactical support from the rooftops).
I think the deal is that that you can just play as either of them. Some missions are designed with their different abilities in mind, but the word is you can choose whoever you want for each one.
It's kind of sad that this would've been a great co-op template for Ubisoft. Two characters that we can play as. One is a male character and the other is female. Both have their own customizable outfits and gadgets. Also, Ubisoft has undoubtedly saved enough money and hired the hardest workers in gaming today to develop female animations. If they started THIS game with co-op, they wouldn't have gotten so much negative press, horrible game aside.
I just have one question, though. How we can play as twins? Did their grandchildren hook up? Great-grandchildren? How can we go through both of their memories without some level of familial relations down the line?
I just have one question, though. How we can play as twins? Did their grandchildren hook up? Great-grandchildren? How can we go through both of their memories without some level of familial relations down the line?
Find the great-great-great-grand-daughter of Evie. Take her DNA.
Find the great-great-great-grand-son of Jacob. Take his DNA.
The twins have likely had separate families, and Helix just has both descendants' Sequences Archived for Abstergo/players. Those descendants probably didn't even know their great-great-great-great aunt/uncle was the other twin. As both were present in the city for the same periods of time, just at different places, selecting either Jacob's or Evie's Genome allows the Helix-user to switch between them. And since they're both in the same place (relatively) and same time (relatively) Helix can just sort of "overlay" both in the same city.
Simple as that, is how I see it.
Yeah, it would have made more sense for co-op to be in a multiple protagonist game, but it also has obvious storytelling benefits.
I'm getting conflicting reports on if you can play as either twin during any mission. Might be that some require one or the other, some let you use either, and all side stuff is by its nature either. The conceit seems to be that they're working together though, I wonder if brotherhood assistance maneuvers will return in a more focused and simple form?