Addressing purely the (list ordered in quality of companion):
- A Spring Druid Sentinel - no punishment, but with tending strike can allow the hunter to pretend to tank.
- Dwarven Knight - very sticky, no healing, will hog the spotlight by virtue of stickiness.
- Vampire
- A Paladin Cavalier - minor punishment, no way to slow or otherwise force monsters to attack. No healing.
Components of the answer, we begin by statting up the characters with options purely from the essentials line enabled in old character builder. I'll also be disregarding recent DDI materiel, even though it can be classed as "essentials" in many respects.
First, a reference hunter at level 1:
Before that, a bit of twitching at the remarkably.. small... list of feats... and weapons... and... wow...
::twitch::
Right, now a drow hunter.
====== Created Using Wizards of the Coast D&D Character Builder ======
level 1
Drow, Hunter
Archery Style: Bow Hunter
Ranger Wilderness Knacks: Ambush Expertise
Ranger Wilderness Knacks: Mountain Guide
FINAL ABILITY SCORES
Str 8, Con 11, Dex 20, Int 10, Wis 16, Cha 10.
STARTING ABILITY SCORES
Str 8, Con 11, Dex 18, Int 10, Wis 14, Cha 10.
AC: 17 Fort: 11 Reflex: 16 Will: 13
HP: 23 Surges: 6 Surge Value: 5
TRAINED SKILLS
Dungeoneering +8, Acrobatics +10, Heal +8, Perception +8, Stealth +12
UNTRAINED SKILLS
Arcana, Bluff, Diplomacy, Endurance, History, Insight +3, Intimidate +2, Nature +3, Religion, Streetwise, Thievery +5, Athletics -1
FEATS
Level 1: Weapon Focus (Bow)
POWERS
Lolthtouched: Cloud of Darkness
Hunter aspect of the wild 1: Aspect of the Pouncing Lynx
Hunter aspect of the wild 1: Aspect of the Dancing Serpent
ITEMS
Longbow, Leather Armor, Adventurer's Kit, Longsword
====== Copy to Clipboard and Press the Import Button on the Summary Tab ======
Giving us a (not counting bonuses) RBA DPR of: +9//1d10+6 or 14.55. And she can therefore drop an equal level monster in 2.2 rounds, qualifying her for the "striker" position. (Don't expect this ratio to last, but enjoy it this level. It just gets more painful from here.)
This build, however, is extraordinarily glassy.
We would therefore want something that can command the monsters attention or block them. And we want something able to endure their attentions.
First, sample instance of the requested cavalier:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 1
Half-Elf, Cavalier
Half elf Power Selection: Knack For Success
FINAL ABILITY SCORES
Str 18, Con 13, Dex 10, Int 8, Wis 10, Cha 16.
STARTING ABILITY SCORES
Str 18, Con 11, Dex 10, Int 8, Wis 10, Cha 14.
AC: 20 Fort: 15 Reflex: 13 Will: 14
HP: 33 Surges: 12 Surge Value: 8
TRAINED SKILLS
Endurance +2, History +4, Athletics +5, Diplomacy +10
UNTRAINED SKILLS
Acrobatics -4, Arcana -1, Bluff +3, Dungeoneering, Heal, Insight +2, Intimidate +3, Nature, Perception, Religion -1, Stealth -4, Streetwise +3, Thievery -4
FEATS
Level 1: Toughness
POWERS
ITEMS
Plate Armor, Heavy Shield, Longsword
====== Copy to Clipboard and Press the Import Button on the Summary Tab ======
First we look to his defender capabilities: how many rounds can he endure being wailed upon?
Average damage per round: 9 * 1-(20-6-1)/20 = 3.150.
Rounds standing: 33 / 3.150 = 10.7
This is just right for a defender. The absolutely critical problem is from his defender aura. There is no incentive for an enemy to remain within the aura and no way for the half-elf to compel enemies to stay, as an enemy who shifts out of the aura sustains 6 whole damage, but can then charge the archer. While it's certainly feasible to have enemies who behave suboptimally, that's outside the scope of this answer.
Sample half-elf sentinel:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 1
Half-Elf, Sentinel
Druid Wilderness Knacks: Herb Lore
Druid Wilderness Knacks: Mountain Guide
Half elf Power Selection: Knack For Success
Beast Companion Type: Wolf Companion
FINAL ABILITY SCORES
Str 8, Con 18, Dex 13, Int 10, Wis 18, Cha 11.
STARTING ABILITY SCORES
Str 8, Con 16, Dex 13, Int 10, Wis 16, Cha 11.
AC: 18 Fort: 16 Reflex: 12 Will: 14
HP: 35 Surges: 11 Surge Value: 8
TRAINED SKILLS
Nature +9, Perception +9, History +5, Arcana +5
UNTRAINED SKILLS
Acrobatics, Bluff, Diplomacy +2, Dungeoneering +4, Endurance +3, Heal +6, Insight +6, Intimidate, Religion, Stealth, Streetwise, Thievery, Athletics -2
FEATS
Level 1: Toughness
POWERS
Level 1 Sentinel At-Will Power: Tending Strike
Level 1 Sentinel Daily Power: Vexing Overgrowth
ITEMS
Hide Armor, Light Shield, Scimitar
====== Copy to Clipboard and Press the Import Button on the Summary Tab ======
Sentinel has AC 18, 35 HP
Wolf companion has AC 16, 17 HP
Assuming the sentinel is tanking, He can tank for 8 rounds and his wolf for 3, not counting healing which will improve those values. His power, Tending Strike, is critical, as it gives the wolf an extra 4 THP per round, significantly improving its tanking power to a theoretical 17 rounds. The difficulty of this strategy is the complete lack of punishment and the active disincentives that both characters have for staying near to them. Of the two, this is a mildly more effective companion simply due to tending strike allowing the hunter to tank for herself.
The dwarven knight:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 1
Dwarf, Knight
Knight Feature: Shield Finesse
FINAL ABILITY SCORES
Str 16, Con 18, Dex 10, Int 8, Wis 13, Cha 11.
STARTING ABILITY SCORES
Str 16, Con 16, Dex 10, Int 8, Wis 13, Cha 11.
AC: 20 Fort: 16 Reflex: 12 Will: 11
HP: 38 Surges: 13 Surge Value: 9
TRAINED SKILLS
Intimidate +5, Endurance +9, Athletics +6
UNTRAINED SKILLS
Acrobatics -2, Arcana -1, Bluff, Diplomacy, Dungeoneering +3, Heal +1, History -1, Insight +1, Nature +1, Perception +1, Religion -1, Stealth -2, Streetwise, Thievery -2
FEATS
Level 1: Toughness
POWERS
Knight at-will 1: Battle Guardian
Knight utility 1: Defend the Line
Knight utility 1: Hammer Hands
ITEMS
Warhammer, Plate Armor, Heavy Shield
====== Copy to Clipboard and Press the Import Button on the Summary Tab ======
Has 38 HP, AC 20, and slow at will. The slow can trigger on battle guardian, which allows him to penalize enemies shifting away. He can tank for 12 rounds, and can second wind as a minor. It is absolutely the best defender to pair with a hunter. Besides a lack of healing, the significant downside to this choice is that a defender of this calibre has the tendency to hog the spotlight: battle must centre around him, and if he's doing his job, no-one's paying attention to the hunter.
The warpriest has mainly the same considerations at the sentinel, being better able to tank, but without what amounts to a walking healing potion. The sun domain provides better tanking capabilities than the sentinel, because it provides scaling resist all, and with sentinel and sun domain, they'd be able to create an effectively invulnerable character.
The vampire has the advantage of lots of forced movement and little need (or ability) for external healing and would make an interesting, if counterintuitive partner.
The thief has remarkable capabilities to hinder enemies with thug's trick and unbalancing trick. It probably would make a better partner than a ranged character for a 2 player game.
In conclusion, you listed two of the top three character possibilities for a secondary character. There is nothing intrinsically bad about a choice of sentinel, though it will not perform a defender's duties like a knight will. By having the hunter focus more on slowing enemies, however, the sentinel can pretend as if he is a defender and try to keep the hunter secure. Be advised that the hunter will not have the spotlight on her simply because any melee character becomes the centre of the battle by virtue of his tactics.
