Muktol

The problem with sideboarding [[Impact Tremors]] is that you still need the manabase to support it. You could try to use mana-fixing lands, like [[Archway Commons]], to get the 1-2 red mana, tough you still would need a way to search for it reliably.

Personally I favour reliability over power. Powerful decks can be fun to play, but they also can be extremely frustrating, when you rely on a single card for your whole deck to work. I normally also play 2-coloured decks for the same reason: The mana-base is stable and affordable. For a competitive 2-coloured deck, the price of the lands will normally make up to 50% of the decks total price. The more colours you add, the more expensive (and prone to disruption) it gets.

[[Altar of Dementia]] could be another option for milling, if you don't want to rely on creatures, tough this cannot be used when [[Heartless Summoning]] is on the battlefield.

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Sadly [[Shaman of the Pack]] isn't available on Arena...at least from what I dug up as information.

+1
Of you want to remove red as colour, you could use [[Syr Konrad, the Grim]]. Personally I'm no big fan of mixed strategy (mill/ burn), tough burn could be a backup-plan if someone uses cards like [[Emrakul, the Aeons Torn]]. [[Consuming Aberration]] would be an option, if you want to go for pure mill.

+1
[[Mage-Ring Bully]] could be replaced with [[Festival Crasher]].

+1
Some ideas, based on my own [[Riku of Two Reflections]] Deck:
[[Rite of Replication]], [[Prophetic Bolt]], [[Rapid Hybridization]]/ [[Pongify]].
Also the magecraft keyword from Strixhaven has some really interesting cards, that could synergise well:
[[Archmage Emeritus]], [[Deekah, Fractal Theorist]], [[Octavia, Living Thesis]], [[Veyran, Voice of Duality]]

There is also the issue that all of your non-basic lands are 4 times in the deck, where only one copy is allowed.

As for the colour identity: The color identity of a card is specified by all mana symbols that appear in its casting cost, colour indicator, AND rules text.
(https://mtg.fandom.com/wiki/Color_identity)

Sorry, I didn't see the "Arena" Tag.

+1
Some things that come to my mind when thinking of discard:
[[Waste Not]] is a controversial card when it comes to discard. I doesn't add anything to your discard-plan directly, but you gain a lot of card-quality or tokens to keep you safe. When using [[Ensnaring Bridge]] the draw can be a little bit counterproductive .
[[Scepter of Fugue]] is a simple but effective card that lets an opponent discard a card if you've run out of hand cards.
[[Megrim]]/ [[Liliana's Caress]] deal damage whenever someone discards a card, tough not much damage for commander probably.
[[Pox]]/ [[Smallpox]] deals with all resources playerds have: hand, lands, creatures and life. Only drawback is that you are also hit by this, but you can plan in advantage for this.
[[Nighthowler]] can convert your small [[Tinybones, Trinket Thief]] into a respectable commander that can easily kill an opponent, especially in multiplayer matches after a mass-removal hit the board.
[[Liliana of the Veil]] easily one of the best Planeswalkers there is... at least for non commander.

A short deck and how to play description would be a nice addition.
[[Preeminent Captain]] would let you play many of your Kithkin spells for free.

Normally such questions would belong into the MTG Rulings & Questions Subforum, but as the notifications don't work for the forum, I will answer you here:
[[Kentaro, the Smiling Cat]] basically tuns the costs for samuri spells into mana of any colour. For example, you want to cast a [[Fumiko the Lowblood]], but don't have any red mana. With a [[Kentaro, the Smiling Cat]] on the battlefield, she just costs {4}.

MTG gatherer, the official database, is always a good tip for cards with rules you're not sure about. Under "Rulings" you will find any annotations written by the MTG team, to clarify the rules for this card. In the case of Kentaro:
https://gatherer.wizards.com/pages/card/Details.aspx?multiverseid=81968
"Kentaro doesn’t change when you can cast Samurai. It just makes Samurai castable for generic mana."
Generic mana meaning mana that can be paid with mana of any color, as well as colorless mana.

I noticed the difference, but with [[Animate Dead]] inly it doesn't make any difference. If you would want to cheat out e.g. [[Emrakul, the Aeons Torn]] you would need somthing like [[Goryo's Vengeance]]/ [[Miraculous Recovery]] or similar, because [[Unearth]] is simply to slow (Sorcery vs instant)

I'm not really convinced on [[Glacial Grasp]]. Stopping a single creature for 2 turns sound OK, but what do you do about the other creatures? I would rather use some additional removal instead because why stop it temporally when you can kill it permanently?

The main problem I'm having with this deck are attacks or damage that can bypass my defence, this can happen in some ways: Direct damage, removal, evasion, swarm or overpower. Or attacks I'm to slow to build up some defence against.
Swarm is the least problem, because sooner or later [[Pestilence]] will deal with swarms and the blockers can still hold of some damage. The problem with evasive creatures usually also can be solved by [[Pestilence]], because they are normally rather weak. Which lefts us with removal, overpower and direct damage. Direct damage obviously is a very large problem, because before using the sideboard I can't do anything against these decks. Removal and overpower deal permanently with my defence, and the itself deck has too little removal to deal with all the threats a deck specialised in larger creatures can put in the field.

When playing I found [[Koma's Faithful]] a useful creature, the mill is rather irrelevant, but the life gain proofed to be useful. Its able to deal with medium creatures and gain some life, at the costs of it's own life. I would rather be looking for something without mill, but low power/ high toughness and lifelink and/ or deathtouch... dream would be a 1/4+ with lifelink (and flying, one still can dream of).

I was also toying around with the idea of using [[Pristine Talisman]] to gain some life, but I don't have anything I can sink a lot of colourless mana into, and the lifegain is too weak. [[Kraken's Eye]] (Not pauper legal, so no option for this deck) would also be ineffective, because getting 1-2 life per turn (More if you're playing against a blue deck) will normally not be enough to balance the damage dealt by your opponents creatures.

"Battlefield", if I would have to guess.

A deck can contain a maximum of 4 cards with the same name. The only exceptions of this rule are the basic lands or if a card's text contradicts this rule. When 2 cards with the same name and supertype "legendary" are on the battlefield the "legend" rule applies:
205.4d Any permanent with the supertype “legendary” is subject to the state-based action for legendary permanents, also called the “legend rule” (see rule 704.5j).
704.5j If a player controls two or more legendary permanents with the same name, that player chooses one of them, and the rest are put into their owners’ graveyards. This is called the “legend rule.”

The latest addition to the deck where 3 [[Pestilence]], 4 [[Curse of the Bloody Tome]] and 4 [[Evolving Wilds]] to help activate the Crab. This helps to keep many but the most aggressive decks at bay, and really increases chances. Yes, the mill is slow, but nevertheless, it's able to do it's work more often now.

[[Deathknell Berserker]] doesn't make any sense, as your only way to increase it's power ([[Unlikely Aid]]) prevents it from dying, and thus gaining the token.
How do you plan to deal with more aggressive decks, or control?

As far as I understand the rules [[Entomb]]ing your creatures won't help you, as all of them have a replacement effect or triggered ability that force you to shuffle it back into your library. This is no "may shuffle them into your library" but you'll have to do so.

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Personally I would remove the 2 [[Griffin Sentinel]] and add 2 additional [[Aerial Responder]]. To make sure you have the right mana you could add some dual- or search lands: [[Evolving Wilds]], [[Scoured Barrens]].
[[Aerial Responder]] was reprinted in the GameNight expansion, where the abilitys are explained with a text.

[[Thornbite Staff]], [[Thousand-Year Elixir]], [[Magewright's Stone]] or [[Illusionist's Bracers]] can be used to double the number of creatures you get from Krenko. [[Thornbite Staff]] also works very well with [[Goblin Sharpshooter]]

I was thinking of making extra paper cards to represent such stack effects. It's much easier to grasp what is going on when you can physically put a card with "Create a Token" or "All creatures get -1/-1" on the stack.

I have added a description on how [[Massacre Girl]] works, because I got the feeling that this card might be complicated to understand. Could you guys read through my "How [[Massacre Girl]] works" description and give me some feedback, if this is understandable? Or how you would describe the function of this card.