Let's talk about Geoffrey Lake, a firm and reliable member of the Telles Royal Guard.
Geoffrey's class is Royal Guard and his skills are Adjustments.
Geoffrey is primarily a physical attacker; his skills don't deal damage, so you'll need to rely on his strength to take down enemies. When it comes to enemies that resist physical damage--or are even immune to it--Geoffrey's attacks become useless.
That is where his skills come into play. With Geoffrey's Adjustments, you can alter properties of allies and enemies. For example, he can reduce the damage taken by the entire party, or increase the damage taken for an enemy. Note that these are not defensive stat changes. They simply take the damage that would be dealt after defense calculations, and changes that amount. As he learns more skills, Geoffrey can alter other things as well, such as reducing damage taken, but preventing healing.
When the Royal Guard Helmet is equipped, Geoffrey will take hits for allies in critical HP. This allows Geoffrey to take the brunt of damage and protect weaker party members. His high HP makes him quite effective at this.
His Level 1 Decisive Skill is Multi-Blade, allowing him to deal a physical attack to all enemies. Simple, but useful. Geoffrey is a good party member, but he isn't without his flaws as well. His Resolve isn't as high as others, meaning enemy skills will make quick work of him. Not to mention his reliance on physical attacks can make later enemies a pain to deal with unless others come to help.