Here’s some back story. I had two PC’s to begin with. All three of us were new to the game. We got the PC’s from level 1 to half way through level 3. They liked their characters and everything was going well. I wrote a quest where they were to rescue an elven princess/Lady from her own castle dungeons after she’s been overthrown by disguised guards. The players got her out of the castle and within the walls of the city, still needing to escape. 

Between this session and the next, another friend heard we were playing and after never playing before, decided he wanted to play too. I started his character at level 3 and it was decided that he’d been told by the owner of the general store (where the lady was hiding) that if he helped the party then trade routes to his hometown could be established. For this next session, I stupidly made the lady a PCNPC (big mistake) 

All three players got her out of the sewers and outside the city walls and that’s where we left that session. Then another friend found out and wanted to join. We had another session where they got her through the woods and to a small town with the promise of horses and a cart in the stables of the town. By this point I’m not enjoying playing Lady Elsys as a character while still DMing. I want the PC’s to have more story with their own characters rather than just babysitting and getting her out of trouble or continuing to the next place. 

My plan for my next session is to possibly have the players wake up to smoke and the smell of burning. Possibly create an overlord type character that has come for Lady Elsys himself. To have a conflict with one main henchman and a few smaller, easy to kill enemies but have the overlord there and get him to grab Lady Elsys, take her away and tell the henchmen to deal with the PC’s while he makes his escape. If I am to do this I’d like him to barely flinch when the players attack him. Making them think that they need to seriously level up and progress their characters before taking him on again but I don’t want to overpower him too much that they’d never beat him even later on in the campaign. 

Any ideas on how I can fix this or if you think the overlord idea could work and have advice on what his properties could be would be great. 

Many thanks, Luke. 

If you want a powerful guy, use the Warlord statblock perhaps. It's like a CR 10, which lv 3 characters could maybe defeat with a ton of luck.

As for having a PCNPC, I mean, you could just be honest about her. Just tell the players, "Hey, I thought it would be fun and not too hard to run Lady as a player character, but it's too much. Would it be alright with you if we retcon and she is still just a weak lady that follows you around?" 

You'd be amazed that most people would go "Sure! No problem."

You could try something as simple as having her fall ill. Something happened in the fights, a small dart or tiny cut from a knife diseased her. The players are now in charge of her safety, she's effectively removed from the fight, and you create a new sense of urgency to the escape. 

Or, if you're set on a "Black knight steals damsel" approach: Give him an ac of 17 (splint), a shield (+2 ac) and a sword he never draws. Have him pull the shield, set his hand on the hilt of the sword, say something condescending "You're not worth dulling my blade..." and then just pull the woman from their group. Then you have Absorb Elements up so you can make him look even more powerful when they cast a spell and then you back-hand them with the same elemental property.

The idea of having her fall ill is an excellent one - make it something quite deadly, and have her ask the PCs to take her to the nearest village or town for medical attention. If they try to medicine check and roll very well, have them alleviate a symptom or two maybe, but she's still getting ill, the symptom(s) come back after a few hours, etc. Then when they finally get her to the town/village/whatever, whoever is helping tells them she'll need to stay there for awhile - there's no guarantee she'll live, and if she does it will take a long time for her to regain her strength afterwards. Weeks, sometimes months. Have her ask the players to leave her there - have her ensure them that she feels very safe there, make the PCs certain that it is a safe place. Away from danger, competent guards, natural defenses, etc. If they need more convincing, have her feed them a plot hook - remembering she heard an interesting story of a unique flower or something that could only be found in some region that, while not very far off, is dangerous, and she always wanted to see one. If they follow it and go to find one of those flowers, turn it into a full adventure of its own. Then when they finally come back - have the city in flames and the woman missing. Lay seeds and hints towards some bigger bad that feeds into more adventure/the bigger plot. Have them unable to find her body - and if you want later, you can say she was eaten/burned alive, or you could even have her appear as a villain later, brainwashed or otherwise controlled or something where the only way to free her from her misery is to kill her in mercy.

Or something along those lines - just a few ideas.

I would NOT go the getting kidnapped route, since it can cheat the players if they have been spending all this time escorting her. Give the party resolution; I'd do this by having a known associate of the Lady having a way to assist her and finding the group. Then the Lady simply opts to leave with this person thanking the party for all the assistance given and to seek her and her friend out in the town/village/city of X. Basically if you have her kidnapped at this point, it has the possibility of making the players feel like you are railroading their choices by making saving this person pointless since it didn't go with your overarching narrative. If you do such a thing sparingly it doesn't risk doing that, but just as players have agency so do NPC's, so if you decide that she would leave with a trusted friend off into some distant land, then that's what she would do.

Honestly, the lady has been rescued.

Have her make contact with a loyal lord and then thank the players for their help with the comment that she may be in touch with them in a few months if she needs any assistance. It may be that she will require help restoring her throne and expelling whoever placed her in the dungeons. It doesn't make much sense that a bunch of third level characters would really be able to offer much help at the present time.

Perhaps the lady can suggest someone to speak with for additional employment. They could work as caravan guards or be sent off to explore something or become agents in her spy agency which is operated by a loyal merchant? Their are endless possibilities. The scenario is logical and it removes the lady from the party which is the basic idea without disrupting the storyline.

The problem with the kidnap/villain scenario at this point is that the characters will usually immediately turn to try to follow and attack the bad guy and rescue the princess again. They will not believe from a character perspective that it is appropriate to leave the princess with her kidnapper for any length of time and it is VERY difficult (and not very much fun) to convince characters that they aren't powerful enough to deal with this opponent so they should just go away until they level up. This can be both meta-gamey and discouraging. So I'd suggest that it makes more sense for the princess to decide to contact some loyal retainers and work to regain her throne while maintaining a link to the characters as a possible future plot point. (Maybe the princess gets kidnapped in a few months and the characters are contacted by the agents of the princess to rescue her again when they are at a point in their careers where they are more powerful).

Hi guys. Sorry I didn’t get back to you all on this. It’s been a mad few months unfortunately with very little time for D&D. I just wanted to say thanks and give you a heads up on my plans. In the couple of games I’ve managed to have, the party went on a small quest to take out a goblin horde for the local town, leaving Lady Elsys in the care of a companion. Upon arriving back to the town they received a letter from Lady Elsys letting them know that she thanks them for getting her to safety but time is pressing on and so she’s continued on to the next city to try and gain some assistance in taking her city back. My next plan is to have my big bad character come to the town that the PC’s are in. He’s looking for her but luckily she’s narrowly escaped. I’m thinking of making him have some monk abilities to make him seem unnatural and almost resistant to their attacks while using that great line of not wanting to dull his sword. He’s also going to be a Drow with disguise person active. Once his concentration is dropped, this will reveal the true enemy. After he manages to find the location of the Lady, he will tell his henchmen to finish the PC’s off.