I'm not gonna lie, I think I may find this to be the most difficult part about writing for me... I find even coming up with characters to be a challenge - in some stories drafts I've just written an "X" for a character and all of their defining visual features, because I didn't know what to do with them... this is actually very bad for writing, as the majority of storytelling is character interaction (even with a single character) and how they look or how they perceive others should have A LOT of impact on character decision making, motivations, actions etc.
I will say that considering I'm more into novel writing now as opposed to comic writing, I don't think I have to be as exact and decisive with character visual design anymore, but it's a good skill to have.
I like the advice that has been given on character archetypes.
Let's take your character Murakami. The majority of characters in popular stories with military backgrounds such as theirs share a fairly distinctive image. Square-jawed, solid man with a buzz-cut hairstyle and maybe some stubble. Likely approaching his middle ages, if not older - so maybe getting peppering in his sideburns, and his skin may be getting rougher with some deeper set lines. Probably prefers function over form so dresses in simple t-shirts; non-restrictive trousers with pockets; and shoes/boots meant for walking. Maybe wears dog-tags. Probably wears a scowl on his face most of the time, and his eyes have a haunted quality to them. Oh, and he probably has a scar or two...
Archetypes like this play off reader expectations really well. This means that when someone reads a discription similar to this/that has parts of this, or see a character designed like this, they can make assumptions about the character and their personality right off the bat. This is good for two reasons 1) you don't have to put as much effort into trying to show their past and experience through dialogue - which often comes across as unnatural self-exposition or takes ages to properly establish - and 2) it makes it easier to subvert expectations with the character without having to contrive super complex plans. This second point is really good when you use the archetype as a base to then build out from or change in a few small ways, to give your character some quirks that seem unique. For example, most readers expect these military types to love animals, especially dogs, but what if Murakami really hates animals, and they really hate him? Perhaps, despite his quite utilitarian view on clothing and appearance, he really likes to indulge in fine dining. Maybe he's now dedicated to covering himself completely in tatoos based on the scars that he has?
Alternatively you can do everything you can to completely skirt around the archetype for great effect also - such as with Kesashi's example of the Hulking barbarian with the battle axe vs the tiny girl with the battleaxe. Bear in mind that this often requires a large amount of suspension of disbelief on behalf of the readers, and therefore you must do the work to set the tone and expectations of the story to properly contextualise the character, so as not to pull the reader out of the experience. Luckily, just the fact that its in a manga helps a lot to set quite accepting expectations based on ingrained preconceptions.
The personality, backstory and archetypal appearance add the first and second dimensions to characters, then the little quirks and subversions, which must tie in to the other things mind you, add the third dimension.
Sometimes though, it is good to bear in mind that people don't innately look like their personalities, although they do shape their appearance to fall more in line with their interests and desired self image. That said, I think, in manga etc, it's quite important to be able to tell main characters apart. Often this is taken a bit over the top.. with crazy hair styles or outlandish clothing, or... top-heavy... body proportions... but one or two specifically defining features - a streak in the hair, a stray lock of hair, a mole or birthmark on the face, prominent eyebrows, an unusual pupil, an earring, a unique piece of clothing like a hat, a scalf or jacket, a unique weapon, a unique accessory like headphones or a rubix cube etc - can go a long way to telling characters apart, and also be used as another visual thing to tell the reader something about a character.
If you make a design choice for a character, especially if it is an aesthetic choice made by the character and not an innate feature, then try to make sure it tells part of their story.