From what an online aquaintance of mine said about Hetalia, I'm assuming it involves anime-style personifications of a country, so let us begin.
TurtleShroom, both as a nation and as an ethnicity, is a hard nation to personify in an anime, for many reasons.
One of the biggest is that TurtleShroomer humans are
absolutely hideous. Anime has the tendancy to make everything look either beautiful or handesome, even farmers, nerds, and outcasts, and that simply does not work in a nation of generally ugly and/or humble looking people.
So let's try and take something, then deconstruct it into TurtleShroom.
We'll start with a picture of a strapping, handesome young man posing as a "nerd" for a television series.We must first remove any feature that really makes him handesome. Make his skin paler, to the point of a sickly appearence, remove any muscle tone, make his teeth severely crooked (or missing, or jutting out of his mouth as buck teeth), and take whatever grace or sexiness he possesses, and destroy it. Enlarge his nose and ears, lighten and congest his voice to a somewhat more nasally pitch, grant a heavy Southern accent, and make sure his glasses are thick, heavy, black, and traditional. Tuck his shirt in, button it completely, and tighten the tie. Drape either a black robe or a blazer suit jacket over him and button it.
There! That's a start.
This seems to be a teenager. All right: raise his height by one to three feet and make him clumsy and far more socially awkward. Give him very strong viewpoints and a
(n) (un)healthy dose of zealous, old time religion. Make sure he's wearing a cross necklace of some kind, be it a small gold chain or a pectoral cross.
Now, change his background. He may look and sound like a nerd, but chances are that his background is working hard labor on a farm. This means he can grow food and work hard, physical labor (with help, since he can't lift anything too heavy), even if he is scrawny and seems to be locked in a basement on a PC every day.
Since this is TurtleShroom as a nation, also remove any skills in technology whatsoever, and replace them with folk wisdom, agriculture abilities, and a thorough understanding of theology. (TurtleShroomian human men that follow Christianity are expected to be walking scholars by the time they're teenagers.)
Ensure that they are extremely innocent, pure, naive, gullible, and a virgin unless married. Do not, under any circumstances, depict the character cursing or performing amoral acts.
Finally, darken his hair, lengthen it somewhat, remove its cleanliness, add heaping pounds of grease and unkempt-ness, and give a decent splattering of acne. Callous up his hands and feet from farmwork and distort his posture.
That should pretty much suffice for a teenage TurtleShroomian Hetalia character.