Hell Girl/WMG

Ai's grandmother is Clotho.
That is, one of the Moirae (the three Fates in Greek mythology), the one who spins the thread of life.

Ai at the end of the first season as a sign of forgiveness.
He built it because he felt guilty for the role he had in, so once she'd come to terms with it, she saw no more reason for the shrine -- a manifestation of Sentarou's guilt -- to exist anymore, so.

In season 2, episode 11 ("The Distant Adjoining Room"), Ren borrowed the pink sweater he was wearing from Shizuko Amagi, the episode's client.
You know, so he wouldn't have to walk-of-shame or anything like that.

Hell is Not Eternal
Maybe this is an attempt of mine to make the show seem just that much more cheerful, but the thing that has been keeping me wondering is that we are never really shown what Hell is actually like. We get a few sequences in Season 1 when Ai gives her clients a "taste of hell", but that was hardly very convincing. There are a few points in between the seasons that we can infer some information from: So why does Ai still include that part in her speech? My guess is that it is an exaggerated warning, as sense of time would indeed become distorted in Hell, and the fact that the sinners will not remember their experience there, Hell would certainly seem eternal. Maybe the Lord of Hell doesn't actually want people to send each other to Hell, and Ai's warning is a subtle white lie to help people change their mind. If this is true, then it would certainly make the series a bit more hopeful, as some of the victims certainly don't deserve to spend eternity in Hell. Or maybe I'm overthinking it, but then again, that's what WMG is for.
 * Season 1: The idea that Hell is eternal is pretty consistant through this season. The gate that Ai and her ferry passes through is clearly a Shinto gate, so for now, let's assume that the hell in Jigoku Shoujo is Yomi, the Shinto Underworld. Yomi isn't necessarily Hell (it isn't a land of pain and suffering), it just is an Underworld of Limbo where the dead goes to, and it is eternal. This would make Takamagahara the "Paradise", which in comparison, would indeed make Yomi feel like Hell.
 * Season 2: This is where the "Eternal Hell" idea becomes a bit more distorted. In the very first episode, the idea of "Repentance" is brought into the series when the Lord of Hell demands Ai to repent for both herself and on behalf of her parents. This would cause the idea of Hell to veer more towards Japanese Buddhism. A major point is that Ai no longer says "Your soul shall forever be lost in pain and suffering, never knowing Paradise" during her speech to her clients. And in Episode 13, it is revealed that people go to Hell depending on "proportions" of their sin (the man will go to hell for both murder and using the straw doll). These points, along with Wanyuudo's rather hopeful speech at the end of episode 5, implies that redemption is still possible even in Hell. This point is further supported in the opening of...
 * Season 3: Despite the fact that Ai says the "Your soul shall forever be lost in pain and suffering, never knowing Paradise" part in her speech again, there is something interesting in the speech and painting of the Opening. In the Opening of every episode, where Kikuri gives a speech, she mentions the "Infinite Hell of Avici". The painting shows various scenes of punishment, such as a man being devoured by dogs, people being beaten by Oni(s) and masses of skeletons impaled or fighting. This is clearly based on Japanese Buddhism and early Taoism. If that is the case, then Hell, despite being very long, is certainly not eternal. The Hell of Avici is indeed infinite, but it obeys the same princicples as all Buddist hells. It is one of the hells designed for a specific group of grave sinners. Most (except for the few who achieved enlightenment) will go to the Underworld when they die, and there, they will be sentenced to a length of punishment by the Enma (lord of the Underworld), in proportion and in form of the sin they committed. After this sentence is served, they will be allowed to reincarnate, and doing so erases all their memories.