It gave Tomoko a nostalgia flashback. It's an adaptation of the 1990s Capcom fighting game series, which she used to play in arcades. (She fought as Felicia.) Imagine a undead-themed version of Street Fighter with more fanservice. It was pretty big and you could see dozens of Morrigan Aensland cosplayers attending the same event. It even got a short-lived Western cartoon adaptation called Darkstalkers in 1995, but this Japanese OVA is much more faithful.

In descending order of importance (here), its characters are:

MORRIGAN AENSLAND (green-haired succubus with mega-cleavage) = often inaccurately portrayed as an evil monster in other media, but here she's friendly and sympathetic. She also gets lots of fanservice, including nipple-less nudity. Her costume's held up by nothing at all and those cosplayers will have needed strategically placed tape. If you watch YouTube videos of the original fighting game, she's always wiggling her fingers.

LEI-LEI aka. HSIEN-KO (Chinese hopping vampire) = has a cleavage window in what's otherwise a modest costume. In the game, she had lots of cool gadgets and weapons, e.g. giant claws, an extensible saw, axes, a defensive wall, a pendulum chain, etc. Here, she's travelling with her sister Mei-Ling to fight monsters.

FELICIA (catgirl) = is technically naked and her "costume" might as well be a few scraps of paint, but somehow she seemed more fanservicey in the game. If judged by how many other video games they've since appeared in, she and Morrigan are easily the most successful Nightstalkers characters. Her special attacks are like Sonic the Hedgehog's, including rolling into a ball, similar kicking moves, a spinning column, etc.

DONOVAN (half-vampire buddhist monk) = the only male character worth a damn in this OVA. The others all have a crushingly tedious motivation and/or personality, which tends to be LOOK ME STRONG LOOK ME FIGHT. Donovan hates the undead, enough though he's one himself. He's on an endless trek to exterminate them.

ANITA (little girl with magic doll) = unclear what's going on with her, but she ends up following Donovan. She doesn't speak, she always seems miserable and she's more dangerous than she looks.

DEMITRI MAXIMOFF (often considered the protagonist of the original game) = I don't care. He's a megalomaniac vampire lord who was exiled to the human realm after... zzzzzzzzz.

LORD RAPTOR, aka. ZABEL ZAROCK (English punk zombie rock star) = he's memorable, but he's also a dick with an ego so massive that I'm surprised it hasn't pulled the Earth from its orbit.

A BUNCH OF OTHERS = they're not interesting here, but the Egyptian mummy (Anakaris) must have been awesome to play in the game. His special attacks are incredible. He can wrap you in his bandages and body-slam you from a distance of several metres away, or else open up his body like a pair of doors and completely envelop you.

The plot's negligible, but it improves as time goes on. Later episodes are less pointless than early ones. Also, the tone, characterisation (and fanservice) are accurate. The animation reproduces the game so well that you can identify a wacky game attack even if you'd never seen it before, because it's so deranged that it could only have been invented by a computer game designer. Meanwhile, the animation quality's so high that it was a feature of the contemporary publicity.

Plot: forgettable. Character design: brilliant. All these unique, instantly recognisable creations... I'd be delighted to get more stories starring these characters, just so long as they didn't have the old scriptwriters. I'd even consider writing them myself.