I No, no "he" here, you are dealing with the impersonal "il", which, as its name implies, has nothing to do with persons.

• Il manque une roue à cette voiture. (There is a wheel missing on that car.)

A better example, as the English will tell even better what is the idea

• It is raining. (You would never think of saying "He is raining".)

II "À qui" is a grammatical word that allows you to introduce a clause; that clause is a relative clause because the grammatical term contains a pronoun that you call relative; in your sentence its antecedent is "fille"; in other words it replaces girl; this way you can rewrite this as follows, but then it is not correct grammar because nothing connects the two clauses.

• Il s’agissait d’une ravissante fille à qui il ne manquait qu’un brin de toilette pour paraitre vraiment sensas.

Here "qui" is "complément d’objet indirect"; that’s required by the verb. However you can use a construction as in English with the subject pronoun.

• Il s’agissait d’une ravissante fille qui n’avait besoin que d’un brin de toilette pour paraitre vraiment sensas. (You just change the verb, and you have your subject pronoun.)

Your translation is not bad, I’d maybe use lack instead of miss.

The pronoun il in il ne manquait is impersonal, it doesn’t represent anyone or anything. It’s just the verbs manquer and "miss" aren’t necessarily used the same way in French and English. e.g.:

Il me manque une carte

I’m missing a card.

See also this reply.

Then, à qui is composed of the preposition à (from) because it is used with manquer here: Il manque quoi à la fille ? and the pronoun qui that refers to the girl, i.e the person who is lacking something.

Instead of à qui, the form à laquelle could have been used:

Une fille à laquelle il ne manquait qu’un brin de toilette.

À qui often translates to "to whom":

À qui de droit → To whom it may concern

Finally, toilette is likely a play of words because it can mean both "grooming" and "nice female outfit".

Here is my translation:

She was just a delightful girl who was only lacking a little neatness to look terrific.

or better, thanks to Luke:

She was a lovely girl who was only a little [grooming|neatness|freshening up] away from being stunning.

Il s’agissait d’une ravissante fille à qui il ne manquait qu’un brin de toilette pour paraitre vraiment sensas.

From San-Antonio Chez Les Gones

In terms of translation into English, here is how to parse the French:

• il manque quelque chose dans la maison = something is missing in the house, notice the reversal in English [impersonal usage]
• quelque chose manque à quelqu’un: someone is missing something, also a reversal in English

il manque in French can be impersonal in the sense that il does not refer to a subject. It functions impersonally. So, to translate you take the object or thing that comes after the verb manquer and make that the subject in English.

Here, the young lady was missing a bit of grooming

à qui: of whom, literally, will disappear in English because it is French structure:

X manque à quelqu’un becomes: X [a person] is missing something.

• Il s’agissait d’une ravissante fille à qui il ne manquait qu’un brin de toilette pour paraitre vraiment sensas.

il ne manquait qu’un brin de toilette=

REVERSAL:

• only a bit of grooming was missing for her to look really sensational.

Note: in English, we don’t use groom much anymore due to the negative connotations re child abuse. However, here it does work.

Other verbs: fixing up, tidying up, etc. In other words, her look was not put together. Most likely her hair and nails.

Now the first part: Il s’agissait d’une etc. cannot be translated using the usual: This concerns or this is about, here. So, let’s take the ravissante fille and do this:

Translation:

1. She was a very beautiful girl who was only missing a bit of grooming (or cleaning up or tidying up etc.) to look really sensational.

2. She was a very beautiful girl who only needed to be cleaned up a little to look really sensational.

The first shows the process but is a bit awkward; 2) is smoother and the term missing disappears, too.

ravissante could also be: ravishingly beautiful but that is more unusual in English.

A bit of a tidy-up is fine too.

In English, you hear: You clean up well, to mean that a person looks nice when they are dressing up.