Where is that from? It’s terrible. I’m a native English speaker and it barely makes any sense to me. The last phrase is a euphemism of sorts for intimate relations.
If I were you I’d throw that garbage out and read something written by a reputable author. Sorry to be so harsh, but it’s truly awful.
The sentences don’t make much sense to me either. I suggest looking for better reading material. The sentences sound like poor translations into English from another language.
I can decifer it, but it is strangly worded. I will put it in plain English and you can tell me if you can understand it.
I think it means:
“Again and again I promised myself that I would explain my feelings, but there was something about the cool, nonchelant air of my companion which made him the last man with whom one could be intimate.”
Do you understand that?
I agree with the other comments that you should probably find something else to read. If native speakers are struggling to understand it, it might be better to focus on something else.
Thank you for that “English” to English translation!
It’s from a Sherlock Holmes novel, “The Sign of the Four” by Sir Arthur Conan Doyle. It was written in 1890 Britain, which explains the flowery, complicated language. That’s just how they wrote back then. 
Sir Arthur Conan Doyle – The Sign of the Four Chapter 1 | Genius
Props to a non-native speaker for tackling such a work!
I read the context around the sentence and it makes a lot more sense now. The speaker was having trouble watching Sherlock shooting up with cocaine or morphine. He really wanted to say something to him, but didn’t think he would be able to speak so freely to Sherlock about it.
Interesting. Thanks for catching that! Recently I’ve been looking for detective novels for language learning. They’re full of basic descriptive language and I thought Sherlock Holmes stories (translated into Korean) might work for me. I’ve never read them and the original English stories are out of copyright. But maybe not!
This is incredibly complicated, but it’s easier with more context.
“Registered a vow” = made a promise to myself
“Deliver my soul” = tell him what I think
“nonchalant air of my companion which made him the last man with whom one would care to take anything approaching to a liberty” = calm demeanor of my friend that made him the last person with whom you would want to risk it
Liberty, in this case, means the liberty (or freedom) to tell Sherlock what he thinks.
Thank you.Maybe I should reduce the difficulty level😂
Yes I like Conan Doyle.Thanks for your sharing.
Thank for your advice.I decide to reduce the difficulty level.
Thanks for your advice.
In LingQ😂Thanks for your advice.I will pay more attention to“BBC 6 minute English“
This is nonsense posing as English. Whoever wrote these words is trying to sound literary at best or is playing a game, at worst. Hope this helps.
It’s from Sherlock Holmes. The problem is it’s archaic.
The last phrase isn’t a euphemism. He’s afraid to take the liberty of saying what he thinks, which is that cocaine is bad.
It can be a euphemism in many contexts. To take liberties often means to be too forward. In this case the forwardness is condemning the others behavior but it is also often used to refer to people being too intimate either emotionally or physically. Here is quote from a Henry Enfield skit that illustrates this usage, “His appreciation for Mozart was a hideous fib. In his eyes, you are only suitable for a sorted encounter in a dark cinema where he secretly desires to take diabolical liberties with your knees.”.
I think your explanation above was perfect. :).
Yeah that’s fair, it can be a euphemism depending on the context. I was just highlighting that in the quoted text, I don’t think that’s what it means.
Also, I think the word you were going for is “sordid.” 
The last phrase is most certainly NOT “a euphemism of sorts for intimate relations.” It “barely makes any sense to” you because you are not a product of Edwardian Britain of the late 19th century, and probably not a regular reader of classic literature.
I agree this isn’t the best choice for someone starting out learning English, but is a story about literally THE most portrayed character of fiction in the English speaking world. The problem is not with Sir Arthur Conan Doyle.
Yes you got it.
The context is that Watson is a doctor and dear friend of Holmes, as well as an admirer of his great mental powers. Watching him take drugs (it is cocaine btw) is troubling to Doctor Watson on all three fronts. He wants to say something, and should, but still doesn’t feel like it’s his place to do so. Yet, he does anyway.