In the climax of a story I'm writing, the antagonist is flying high in the sky(she has wings) during a thunderstorm, while wearing a full suit of armor. She is struck, and nearly killed. But lightning is only attracted to things touching/connected to the ground. Could being attached to the ground(through a rope tied to her ankle or something) make it work? She is the only one injured by the lightning strike.
As far as lightning is concerned, everything is connected to the ground.
Lightning is a brief one-way current from the ground to a cloud (or sometimes the opposite way). That current is going to preferentially go through good conductors (like metal armor and humans) instead of bad conductors (like air). Lightning can go through air, obviously, but if there's a large conductive object in the vicinity, it's probably going to go through that instead.
Presumably, your character doesn't have armor covering her wings. In that case, I think the most likely path for the lightning strike to take is to go in one wingtip and out the other. She's going to have a bad time. Even if her armor is made of metal, it isn't going to help her here, because the lowest-resistance path from one wingtip to the other goes through her body, not through the armor.
If your character is much luckier, the lightning strike could go entirely through the armor—perhaps into the helmet and out through a shoe. In that case, I think that she'll be totally unharmed by the discharge itself. A lightning bolt only carries about 15 coulombs of charge, and the worst that 15 coulombs will do to a large chunk of metal is to heat it slightly.
The armor won't change the fact that a lightning bolt just struck through the air above and below her. I don't know what the effects of that on her would be; at the very least, it would make an extremely bright flash of light and an extremely loud sound.
(The protective effect of metal armor is sometimes described as a Faraday cage effect. I don't think that's accurate. A Faraday cage blocks electromagnetic radiation by completely enclosing something. A lightning strike isn't electromagnetic radiation, and armor doesn't have to completely enclose you in order to have a protective effect—theoretically, a single wire would work just as well. So, a suit of armor may protect you from lightning, and a suit of armor may act as a Faraday cage, but those are two distinct and not-very-closely-related properties of a suit of armor.)
A lightning discharges through the path of minimum resistance.
In a process not well understood, a bidirectional channel of ionized air, called a "leader", is initiated between oppositely-charged regions in a thundercloud. Leaders are electrically conductive channels of ionized gas that propagate through, or are otherwise attracted to, regions with a charge opposite of that of the leader tip.
The positively and negatively charged leaders proceed in opposite directions, positive upwards within the cloud and negative towards the earth. Both ionic channels proceed, in their respective directions, in a number of successive spurts. Each leader "pools" ions at the leading tips, shooting out one or more new leaders, momentarily pooling again to concentrate charged ions, then shooting out another leader. The negative leader continues to propagate and split as it heads downward, often speeding up as it gets closer to the Earth's surface.
Once a conductive channel bridges the air gap between the negative charge excess in the cloud and the positive surface charge excess below, there is a large drop in resistance across the lightning channel.
A large electric charge flows along the plasma channel, from the cloud to the ground, neutralising the positive ground charge as electrons flow away from the strike point to the surrounding area. This huge surge of current creates large radial voltage differences along the surface of the ground.
A connection to ground is not strictly needed, as a human body can be less resistive than the equivalent length of air, acting therefore as a preferential channel for the discharge.
It is somewhat similar to how airplanes get sometimes struck by lightning despite not being wired to the ground.
For sure having a physical connection to the ground will make it for a higher chance of being hit, if it would lower the resistance of the path (e.g. wet or conductive line)
Being hit: Plausible. Surviving it: Plausible?
You might be interested in the story of Zhongpin He, who was sucked into a storm while paragliding and later found dead. Lots of things could have killed him but post-mortem examination revealed he was killed by a lightning strike.
Apparently 9 out of 10 people survive lightning strikes, though injuries can be varied. The biggest danger is cardiac arrest and no one around to deliver CPR.
I couldn't readily find a story of someone being struck in the air and surviving but I don't see why it would be worse than being struck while on the ground (other than the no CPR thing, but not all lightning strikes result in cardiac arrest either -- some end up with no apparent injuries at all). Probably just a roll of the dice if it kills you or not. Burns are a common injury, but of pretty random extent (I found stories of anything from no burns to 3rd degree).
Since we know it can strike paragliders in the air (that's where you're just kind of sitting under a parachute type thing), and we know it's possible to survive a lightning strike, I think your scenario is plausible, and you can give her injuries of anything from severe burns to literally no problems at all.
Lightnings routinely hit airplanes while high off the ground. This is generally safe and the worst damage is some paint getting burnt. If something goes wrong, this is because the pilot is temporarily blinded or disoriented.
If there are conditions for "almost" create a lightning, a conductive object near the possible lightning path can facilitate the electrical discharge by shorting the path as well as concentrating the field lines near its extremes.
From the lightning viewpoint, a bare human body is almost as good a conductor as a metal piece. Wood, concrete, wet stone, chimney smoke - everything is better than air in this regard.
No need to tie your antagonist to the ground in order to get lightning, but it can help a lot. It will be a very high lightning rod.
In regard to survivability:
I can't imagine how wings and full suit of armor play together, but if the wings are covered with metal, we are generally in the airplane hypothesis where the metal protects what's inside very well.
On the other hand, touching the same metal in more than one point and/or bad electrical connections between the metal parts can make the lightning strike over the metal deadly or at least very much shocking (because of the voltage drops over the resistance). There are simple measures that can get your armor lighting-proof, but these should be done in advance and require knowledge.
Even if everything else goes well, the metal parts will get rather hot to touch (20..100kA of the garden variety lightning are not a trivial engineering task) and even worse near joint points. Parts may simply weld to each other in the joints. Not good for aviation.
And then... there are quite a few things in a thunderstorm cloud, besides the lightnings, that can kill someone. Like, e.g. gusty and turbulent winds that can stress your body mechanically up to tearing it apart. Ice pieces in the few kilogram range (pretty much normal, this is what hail looks like when inside the cloud core) aren't safe either.
Related solution to the wing-armor-survivability issue is to just have metal covering over the main joints of the wings that connects to the rest of the suit, kind of like flashings but for wings.
It would allow the current to never actually go through the wings, because the metal armor will attract the current better than wet plumage.