Mechanically, Keris' soul has "mortal" animals (technically, Enlightenment 0 akuma, because an akuma is a creature native to a Primordial Mythos) because the Devil Domain and Tiger Empire can be shaped by her souls by "Wyld Shaping" within their themes. Wyld Shaping naturally makes animals to populate a land, so there's no reason you won't get weird animals that are basically there to be eaten by the demons and add a bit of fluff to the place.
(My rule of thumb is that most Infernal souls are basically going to wind up looking like Pokemon AUs because of the mix of demons and akuma wildlife.)
Especially in the Swamp, because Haneyl and the sziromkeruby are relentlessly carnivorous. Sziromkeruby ecologically basically fill the same role in Keris' soul as foxes do in Creation. A sorcerer who summons a petal-cherub as a familiar will find that they keep their kitchen clean of mice and rats - admittedly by spitting fire at them, so it's probably a good idea to make sure your sorcerer's tower is made of stone. Then they'll probably start depopulating the surrounding countryside, like cats would if they could spit fire.
Actually, building off this, let's have a look at the diets of the various keruby breeds! Some of them have undergone a few changes since they were originally posted in this thread, and will be getting minor rewrites - sziromkeruby need their predatory cattish sides given greater prominence in their bio and femkeruby need to have their un-taming thing toned down so that they're more useful as familiars, etc.
A few of these changes may be apparent from the following.
Szelkeruby
Szelkeruby are haemovores, and require blood to survive. When they eat raw meat they can get enough in their diet, but otherwise they have to hunt down and drink a mouthful every few days. The tongue of a wind-cherub can close open wounds, and so a szelkerub blood-hunt usually involves a pack tracking down their prey, slashing at them with knives to get access to the delicious blood, and then licking the wounds to close them up and send the still-alive prey on its way.
Otherwise, the wind-cherub diet is relatively flexible, and given blood they can survive nutritionally off a diet of grassy seeds, insects, fabric and salt. However, szelkeruby adore sugar and will do almost anything for it. The border with the Meadows is marked with a massive export trade of honey for the hungry szelkeruby of the Ruin, and sugar-cane toads are kept under control by relentless predation.
Orvenkeruby
Wave charmers are of the Sea, and such is their diet. Dry food of any form is indigestible for an orvenkerub, and forcing it down can damage their throats and leave them with terrible stomach pains. Their preferred meals are bowls of fish and seaweed in brine - they find most aquatic plants and animals to be edible, and see little difference between the two in their meals. When away from easy access of water, orvenkeruby face dietary challenges. They are able to eat land-grown food if adequately soaked, but dislike the taste. Freshwater is less palatable to them than brine, but they will catch rainwater in bowls to soak their food in if they must.
Demonologists might be surprised to see a wave charmer eat, for they devour huge portions despite their childish stature. A hungry orvenkerub can pack away twice or even thrice as much food as an adult human, and then be content with only water for several days thereafter. Rice and salt soup is surprisingly agreeable to their constitutions when on land; perhaps a result of the flooded fields rice is prone to grow in and the sheer size of the portions they favour.
Sziromkeruby
Onlookers have noted that the eating habits of the petal-cherubs much resembles that of the fox or the cat. Sziromkeruby are relentless omnivores, who prey on anything smaller than them - or larger than them and unable to adequately fight back. They are unable to digest raw meat and so must cook it in their fires. This they do gleefully, and the entire species loves cooking only slightly less than they love reading and writing. They also eat vegetation and paper, although they grow sick if they cannot get at least some flesh in their diet.
The tastebuds of the sziromkeruby are acclimatised to the herbs and spices of the Swamp, and so they will tend to find the food of Creation bland. Petal-cherubs who travel outside their homelands will often carry seeds of their favoured spices with them - which can often also be used to make inks. Humans should be wary of food spiced to a level a sziromkerub prefers, as mortal tongues will likely not be used to such painfully intense flavour.
Viharkeruby
Cloud thieves are obligate carnivores. They eat their victims and have little problem digesting anything which was once living. Their predatory nature is aided by the fact that they easily fall into hibernation. When dozing like that, they still listen and can wake up with just a few minutes of noise. Within the Edgeland mists they hang from branchless trees by their feet, dropping silently onto prey that stumble blindly through their cloudy domain.
Viharkeruby are also ice-eaters. They like the taste, and must gnaw and scratch at icicles or frozen surfaces to keep their teeth and claws from becoming blunt. One of the signs a cloud thief is in the area are the marks of teeth or claws on ice. If there is no ice, the same markings can be found on stone which they treat as an inferior substitute.
Zenekeruby
A lyrical heir has a two-part diet. As they describe it, they must nourish both the music and the instrument; feeding the mind and the body in equal parts. The former is simple - a zenekerub feeds in part from the music of others. This is why they occasionally group together, and why they favour angyalkae. Music played poorly sickens them, and they will attack those who sing or play an instrument with insufficient skill.
An interesting result of this ephemeral half of their diet is that zenekeruby appear to eat little. They consume wax and rosin, and can often be found chewing on candles, as well as chell-meat and certain oils. Of all the keruby, lyrical heirs feel the bite of hunger least, and must sometimes be coaxed or reminded to eat by their followers when caught by musical inspiration. If they are playing, other keruby will too busy dancing, and it is not unheard of for zenekeruby to faint mid-melody if they are too devoted to their performances.
Mezkeruby
This is the paradox of the mezkeruby - they are herbivores and tar-drinkers who nevertheless must periodically eat meat, once a month or so. If they fail to do so, they become neurotic and clumsy, growing more and more deranged and uncoordinated the longer they abstain. Just a mouthful of meat will clear up these symptoms in minutes. Tar augurs hate the taste of meat, and try to cover its flavour with tar and oil and honey whenever they must take it medicinally. Occultists speculate that this is some consequence of their kerub heritage - that the breed is 'meant' to be omnivorous and through abstaining some essential facet of their essence does not function correctly.
Otherwise, the mezkeruby have a largely liquid-based diet. The tar and honey from their homeland along with long-grained rice and various bog-plants are their preferred diet. The honey of the Meadows is often bitter, and so the tar augurs learn to love bitter things and eat the poisonous plants of Creation with apparent glee.
Femkeruby
Even compared to the sziromkeruby, the femkeruby are ferocious omnivores. A spark jockey will eat anything that it can force into its spark-filled gullet, and unlike their petal-cousins they can digest rock and metal. The garbage heaps of Krisity will usually have the local femkeruby picking over them, selecting choice morsels. Femkeruby drink beer over water if given a choice, and so are frequently tipsy. The breed has no sense of taste as mortals know it, and so anything that provides nutrition is acceptable to them - but nothing more.
However, their greatest delight is lightning, which brings all the sensations that they lack in taste into vivid, blissful clarity. A single strike can keep a spark jockey fed and buzzed for a month. Liquid lightning, refined in the Spires, is their drug of choice as it stimulates them to incredible heights. It is taken as a glowing blue fluid, or else in the form of tiny pills of charge-holding metal.
Angyakeruby
The living canvases - as might be guessed from their name - consider art ingredients of all sorts to be food. To this they add the coastal foods of their home islands. One will happily sit down to a dish of shellfish, ceramic and samphire flavoured with a fine oil-paint sauce, served with a shot of lacquer - though lacquers are intoxicating and get them outrageously drunk. Their nature makes them inordinately fond of hallucinogens, and visitors are advised to take care when eating a meal prepared by a living canvas.
Angyakeruby are artists, and some choose to specialise in cooking. In their homeland, such living canvases will often wind up feeding tens or hundreds of their siblings at once. Cooks are powerful artists within the Isles of Krisity, and if they are capable they are paid well in ingredients and works.