In both dramas there is an adorable leading lady and an equally adorable mother of the leading man. The mother goes to great lengths to match make for this couple..
Differences:
Itazura na Kiss' leading man is the typical cliche smart and handsome Jerk. He does NOT want to admit that he likes the girl, even to himself. The leading lady is adorable and sweet. She is not very talented but determined to better herself.
My Mr. Mermaid's leading man is dedicated to his sport but kind and sweet. He seems quite smitten with the leading lady, right away. The leading lady is dedicated to becoming a good reporter, but equally dedicated to being an honest upright person and good friend.

Itazura na Kiss is based on a manga and there were versions made in Taiwan, Korea, Japan, Thailand, and China... I loved them all. However, if you do not like going through all of those episodes, that make it seem like the girl is chasing a guy who is just not that into her, try Kiss Me(the Thai version) first. In that version, it is apparent, very early on that he has a soft spot for her, compared to the rest.

Both are fantasy dramas Both have adorable, somewhat mischievous leading ladies.
Both have an element of the couples uncertain future, since they "from two different worlds." One is literally a different world and the other is figuratively, but that fact threatens the happily ever after of both couples.

The leading man, in both dramas, is sought after by many women but he show zero interest in any, until he meets the leading lady, who is like a breath of fresh air. The leading lady, in both dramas, are adorable, energetic, and independent.
The leading man is decisive and is the pursuer. Since the leading men in both are irresistible, the leading ladies fall easily for their good looks and charm.
Even though Love O2O is not a fantasy drama, it does have a fantasy element, with the video game they play. The game element also allows this modern day drama to include a bit of the traditional Chinese clothing and hair styles.
Main differences: Be aware that in The Eternal Love there are some really cheesy special effects.The Eternal Love has a backdrop of Royal politics/Sorcery.
Love O2O has a back drop of social media/College-life/business.

Both are Drama/Romance set in a workplace.
There is a similar dynamic in both between the male and female leads. In both the male lead is the direct supervisor/mentor/teacher of the female lead. In both the female lead is talented and competent.
In both the couple are equally attracted to each other(as opposed to those dramas that someone is the pursuer). The differences are in the actual plots.
Les Interpretes deals with disapproval of others about what seems to all an inappropriate work relationship and professional reputations. Heirs deals with legal issues that relate to inheritance law and the secrets each of the leads family have kept from them.

Both have corruption in law enforcement.
Pride and Prejudice: is faster moving with more action. The prosecutor trying to uncover the man who cover up the murder of her younger brother. She believes it could only have been done by a fellow prosecutor. Eventually she gets help from co-workers to solve this case after realizing it is connected to a couple of other cases that have come across their desks.
Stranger: is developing the story a little more slowly. One cop and one prosecutor work together after realizing they have no one in their own departments to trust. The case that brings them together may be related to higher ups in both of their departments. The prosecutor is unable to feel emotion and is balanced out by the cop who is motivated by her feelings of sympathy for the victims and victims families.

Both dramas feature time travel to a different era.
In Tunnel, granted it is just from 1986 to present.(still a big culture shock)
In Faith it is to the Goryeo era from the present.
Both bring a little star power with their Brave and Manly leading men.
Faith's cast is lead by Lee Min Ho playing a General of the army and bodyguard to the King.
Tunnel's cast is lead by Choi Jin Hyuk playing a tough Detective who is not afraid of tackling a dangerous criminal. He is also a respected sunbae for his less experienced partners.
Both have a lead who is faithful while longing for their loved one after being separated by time.
Both are directed by Sin Yong-hwi
Both are really good dramas worth watching.

Where they are different:
Tunnel has a serial killer crime story and slightly dated outfits.
Faith has political intrigue, medicine, and of course the long hair and historical dress.

Both are well done Chinese Fluffy romance dramas. Neither rush the story, so the relationships seem to evolve more naturally. Both feature a Boss/employee relationship. Boss and me is a little more traditional, with the Rich handsome man falling for the poor young spunky girl. Across the Ocean To See You is a bit of a modern twist, the male is the subordinate and the the boss is already pregnant when they meet.

Both are Makjang Dramas. Both have a wealthy family that is tied to a poor family. Both have a main character that deals well with a disability and plays piano. In Can You Hear My Heart it is Deafness and in You Are Too Much it is Blindness.
Differences:
You Are Too Much will be 50 episodes and is categorized as Drama, Family, Music - Music business is featured
Can You Hear MY Heart is 30 episodes and categorized as Family, Melodrama, Romance. - Romance is featured
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The Mom's in these are very similar. Both seem way to excited about getting their son together with a girl that most rich drama mom's would be trying to pay to stay away. Has some similarly corny characters and overacting. Her childhood friend, who is always around to have her back, is very similar to the second lead. there is also kind of a class A class B rivalry. The only real difference is that the leading lady, in Master Devil Do Not Kiss Me, is not head over heels for the leading man. It also appears to me that he likes her first..
Both are fluffy, fun, and adorable.
Note: I watched Master Devil Do Not Kiss Me, right here on MyDramaList !!!

The Mom's in these are very similar. Both seem way to excited about getting their son together with a girl that most rich drama mom's would be trying to pay to stay away. Has some similarly corny characters and overacting. Her childhood friend, who is always around to have her back, is very similar to the second lead. there is also kind of a class A class B rivalry. The only real difference is that the leading lady, in Master Devil Do Not Kiss Me, is not head over heels for the leading man. It also appears to me that he likes her first..
Both are fluffy, fun, and adorable.
Note: I watched Master Devil Do Not Kiss Me, right here on MyDramaList !!!

The Mom's in these are very similar. Both seem way to excited about getting their son together with a girl that most rich drama mom's would be trying to pay to stay away. Has some similarly corny characters and overacting. Her childhood friend, who is always around to have her back, is very similar to the second lead. there is also kind of a class A class B rivalry. The only real difference is that the leading lady, in Master Devil Do Not Kiss Me, is not head over heels for the leading man. It also appears to me that he likes her first..
Both are fluffy, fun, and adorable.
Note: I watched Master Devil Do Not Kiss Me, right here on MyDramaList !!!

Both have a female ghost main character, a main character that can see dead people, a past their prime Shaman type supporting character, and a sweet character with a dark secret. Also, the ghost is trying to remember what happened to her and figure out what she has to do to move on, in both.