#smprg

They are both highly and openly emotional. Both characters are sobbing when we meet them in the story and afterward cry so frequently/powerfully that it's a defining character trait.

Both also have character arcs in which they start off as immature, with low self-esteem, and without direction in life, but become courageous, motivated and strong by the end of their journey

Both characters' lives are profoundly impacted by absent family member/s

Cute bangs

Raised by someone with conservative/traditional values

Both are distinguished in their parties with a light green symbol (Yoosung gets the light green heart, Mallow's magic spells are proceeded by a light green ♣)

Both are gullible

Light blue clothes

Uhhhh if you want to count Mario's unreal miming skills in SMRPG, then I guess both Yoosung and Mallow have someone in their life who strongly reminds them of someone else

Both are raised by the water¹, and with hard work, take their place in the SKY

Mallow has healing magic, Yoosung studies medicine

Money problems

Both favor striking weapons (Mallow's froggie stick, Yoosung's giant hammer in LOLOL)

Both have parents with government jobs

Out of the men in their party, they are the shortest. Out of all the party members, they are physically the weakest (Considering Mallow's Attack stat is lower than even Toadstool's, and the fact that Jaehee does judo and probably has the edge on Yoosung there)

Although there are no canonical ages for the SMRPG party members, I think it's fair to infer that Mallow, like Yoosung, is also the youngest party member (with respect to Geno's age as a star rather than his time 'living' as a doll)

Both have relative/s who were in charge of a group of people. A artistically inclined green(ish) haired person lies about the health/wellbeing of Yoosung’s/Mallow’s relative/s, and assumes authority over the group. The RFA and the citizens of Nimbus Land both accept the lie, but Yoosung and Mallow persist in skepticism of the usurper's claims

(Why yes, I did just compare V to Valentina. Let that mental image marinate in your head.)

Bus rides

Both defeat dragons, with the help of teammates

Omelettes are fluffy, much like Mallow. (Also with regards the MM April Fool's event, you could also argue that both characters have a reason to dislike birds)

Both of them have a struggle with a switch toward the end of their stories

Mallow has a spell that summons a falling star, FFS