The Doctor is able to make them visible using a localised gravity field. The Here appear as a swarming mass of fireflys but the field causes them pain and they beg the Doctor to free them.
They are able to speak with Maebh, possibly using her psychic connection. While she speaks the deep voice of the Here is also heard. They speak as a collective indicating a hive mind.
The Here identify themselves as the life that prevails. Eventually it is revealed that they are responsible for the trees growing over night and that they have done this in the past. They also take credit for plants growing between cracks and on mass graves.
This would suggest that they are linked to plant life, able to transform themselves into what ever is needed. This is supported by the end of the episode where the trees, having done their job, vanish in a mass of gold light. This would explain where the mass of the plants came from. Rather than growing organically the Here just converted their own bodies.
Their function seems to be to protect the Earth. This might not necessarily include the human race, since they obviously have a low opinion of the human’s capacity for death and war.
This could explain why the Here haven’t made their presence known before, for example preventing prior attempt to invade or wipe out humanity. They only act when the planet itself faces destruction.
The Here are obviously proud of being around for a long time but it is curious that they are unfamiliar with the Doctor, given how integral he has been to saving the planet for thousands of years.
It could be that the Here are a relatively small group (although they obviously they are spread far enough across the globe to transform the whole planet). It might be that they are not that intelligent, and without someone like Maebh they can’t communicate or understand basic concepts.
While the Here make reference to saving Earth in the past it appears that they were called there to deal with the crisis. In this case it is the solar activity of the sun which has summoned them. It could be that they aren’t native to Earth and travel through the universe, aiding whatever planet needs them.
The Here are also confident that they will outlast everything else. This would indicate some form of immortality. If they are a hive mind it could be that their physical forms are mortal but their consciousness continues as long as their can reproduce.
Rather than immortality the Here could possess some degree of future vision. They might believe that they will outlast everything because they can see it. This could also explain why they knew about the solar flare when even the Doctor was unaware of it.
In this adventure the Here helped to save humanity. In the future they might become their enemy. In order to protect itself a Here summoned tree denies a fire oxygen. If the Here decided that humanity was a threat to the planet they could create forests and similarly prevent oxygen from being released, suffocating everyone.
This could lead to a Doctor Who version of the film ‘The Happening’ or ‘The Day of Triffids’. Depending on the degree of control the Here can exert they could create plants that release toxins, hallucinogens or bring plants to life to eliminate humans. If the Here are everywhere they could spy invisibly on the PCs, anticipating their every move.
In such a scenario the PCs might need to seek out a telepath to communicate with them, possibly using a gravity field to facilitate this. If all else fails these gravity fields might be used to eliminate the Here, although this would leave the planets’ ecosystem defenceless.
If the world was in danger the PCs could seek out the Here as potential allies. They would need to persuade the mysterious race that any invaders threatened the whole planet. If successful the PCs would have a powerful ally, capable of controlling the environment and plant life.
An alien race might target the Here specifically. The species could be captured and studied to identify a weakness. PCs would need to stop the experiments and prevent any weapon being used against the Here for fear of leaving the planet vulnerable.
In the future it is possible that a dialogue could be established with the Here. Humanity could show themselves as willing to protect the environment and work with the Here to protect their home.
This could be a great period of regrowth and reconstruction, while the rest of humanity continues to explore space. What knowledge and insight could the possibly immortal race of the Here share?
We know that Earth has a finite lifespan. What happens to the Here then? Did they survive until the final days? Would they die with the Earth or move on to other planets that need them?
The Here are like the opposite version of Vashta Nerada. Both are near invisible swarms with a connection to trees but while the Here associate with life the Vashta Nerada embody death.
Is this a coincidence or are they related? They could be different evolutionary branches of the same species or one or the other group could be a rogue faction. An adventure exploring this connection could place the PCs between the two groups.