Rank needed for officer, please

Good morning

Some of you may remember that I recently posted a query about musket injuries: https://www.thenapoleonicwars.net/forum/peninsular-war/musket-injury-information-needed

As you were all so very helpful, your reward is to be bothered again with what will seem to you a very simplistic question. But as I have no military background at all in my family (we're brewers and butchers - both handy in their own way), I hope you will forgive me.

In short, I am writing a series of books whose narrator has a background as a groom/servant in the Peninsular Wars (with the 48th Northamptonshire). He goes from England to Gibraltar, Spain, India and Australia with them, all the while working for "his officer" as a groom. He eventually accompanies "his officer" home to Cambridge, where the officer dies of syphilis and my narrator starts his life as, well, the narrator of a series of books set in Cambridge (and an ostler - always with the horses).

Sorry - that wasn't very short, was it!

My question is this: what rank would "his officer" have had? To fit my plot requirements, the officer is aged 22 in 1809 in Spain, aged 24 at the Battle of Albuera (where my narrator gets the injury mentioned in the other forum post), and returns home to Cambridge to die in 1824, aged 37.

Ideally, I actually have three questions:

1. What rank would the officer have had at the Battle of Albuera (1811)?

2. Would he have been promoted in the following years - i.e. 1811 to 1824, when he was invalided home?

3. What would my narrator have referred to him as? Captain [or whatever] Name? The officer? My officer?

Thank you all so much - I am determined to get as much of this right as I can.