Merlinus is horrible. He's more useful in Fire Emblem 7 since he gains levels every Chapter even though you have to take a lot of time to get him to turn into a Transporter. In Fire Emblem 6, he starts as a Transporter but he can't gain levels unless he is attacked 100 times. As such, my Merlinus didn't gain any levels. His level 1 Transporter stats are way worse then his younger form's level 1 Transporter stats. I'm also quite sure you can't redeploy him once he dies, but I never opted to use him and never got him killed. Another bad point about him is you can't choose if you want to deploy him on top of the characters you pick. You have to choose to NOT use a character slot and instead bring him along if you want to use him.
Method to Acquire: Eliwood orders him to accompany Roy. He starts with you on Chapter 2.
Damage: N/A
Critical: N/A
Speed: Nothing special.
Dodge: Horrendous. He can be destroyed very easily.
Rating: 8.85/10. He can't be put in any square that an enemy can move and attack. While I hate his guts, he's exceptionally useful because he can hold a lot of items for you. This makes him far better then many characters.