It honestly depends on how you are writing the web series. How this character dies should also bring about a whole plot line.
Nuclear radiation - Why? What accident happened? What changes in safety regulations?
Sci-fi disease - Why? Was it because of first contact? Does that hinder the relationship with that alien creature?
alien creature - Why? Are they just big baddies who don't have any motivations of their own except to be a big bad or is this an alien creature mob boss and they had to kill this person to keep a secret safe?
Lots of places you can take this death but overall it should reflect what you are going to be writing.
Be aware how slow and gruesome dying of radiation poisoning can be unless the dose is immideately fatal. including a day where they feel much better, the Walking-Ghost-Phase
This would be a massive burst from all directions, absorbed through a faulty radiation suit. In my rough plan, the protagonists find a secret research station underground, where experiments were run involving delta radiation (which is a real thing, look it up).
I would probably not do a disease unless you are are trying to evoke feelings related to covid, or it's very, very different in symptoms. (and maybe not even then) - only because I feel like that's carrying a lot of baggage right now for some folks.
What is the most dramatic way? Could another protagonist die from it when one of them chose to save the day instead? Can it be a daily ocurrance, and that guy was just the unlucky one?
Was it something just utterly mundane like having their feet stuck in something? Does that guy have actually to die to finalise their character arc in a satisfying way?
Nuclear Radiation almost killing people is probably the most problematic, there will be all sorts of long-term damage.
The sci-fi disease at least has the exact properties you want it to have, and the Alien Creature option simply gives you physical damage which comes in your choice of severity. I'd lean toward that over all, but it really depends on what fits the story best.
And might I recommend Royal Road as one of your publishing platforms? Pure fantasy tends to do better than Sci-fi, but at least it is a point of traction. I know a lot of people post across multiple platforms.