How do you write a character that we've not even seen in flashbacks?
Very simply, you refer to the source material. We already know that Nigel Griffin was The Invisible Man. The problem, of course, was whether to use HG Wells' novel or the League of Extraordinary Gentlemen as my source. I know Damian Kindler is a great fan of TLOEG and so I was tempted to head in that direction. But there were hints that suggested otherwise. Firstly, it is Nigel Griffin in Sanctuary, and not one of the other characters from the comics. Basically, in the novel (for those that aren't familiar with it) Griffin's first name is not revealed. Secondly, he was a medical student at a respected English university (which ties in nicely with the idea of The Five at Oxford). There is also the fact that owing to the vampire serum, there is no "albino decoy" as the comic books suggest. Lastly there are slight hints in the writing of Sanctuary. Clara (Griffin's granddaughter in the show) says that she can "dip into any till she likes" (paraphrased). In HG Wells' novel, Griffin is a theif.
So, having decided to use HG Wells' novelisation as a source material, what implication does that have in terms of writing? Obviously, we can't adhere too strictly to the novelised character, as we know the photosensitive molecules to be a result of the vampire serum and not the potion in the novel. Furthermore, we can assume from how Clara uses the power, that the invisibility was not a permanent state, as with Griffin in the novel, but rather something that could be used at will.
The first thing that it can help us with is physical appearance. In TIM, Griffin had a rare physical condition known as albinism. So we know that he had extremely pale skin and white hair. Contrary to popular belief, it is very rare that those people with albinism have red eyes (unlike in animals). It is more normal for them to have very pale blue eyes, although they can take on purpleish hues depending on the blood vessels underneath.
In terms of personality, he was extremely hard working and studious, though often overlooked. This would indicate that he was somewhat mild mannered and quiet, at least at first. It was only after he turned invisible that he became mad, which would fit in with the mythology of The Five (three went completely cuckoo, which would cast some interesting doubts on Helen's notion of her own sanity). He began to steal when money got short as well.
Using the comics as a sort of transparent overlay, you can build up even more facets of the man. For example, Griffin in the comics used to go to a girls' school and rape the pupils there whilst invisible. This would tie in with the insanity, both with that of the character in the book and with Tesla and Druitt who could hardly be called sane following the vampire experiment.
So I began to build up a backstory for Griffin based on these things. He went to Winchester College as a boy, then to New College, Oxford, where he read medicine (Helen and James both attended the same lectures, and Druitt was a scholar at both Winchester and New College). He came across Tesla through his interest in physics (an interest documented in the novel).