Question: the Divine Companion ACF does not mention what happens if I cast a 0-level spell on the companion. How much energy does it absorb from the spell? None? There are a few effects that make 0 level spells count as 1/2 level.
Divine Companion description for reference:
By sacrificing a possible link to an earthly creature, you gain the ability to store arcane energy in a divine reservoir that takes the form of a spirit creature. Linked to you through your own life essence, this divine companion can then transform that energy into beneficial effects.
Level: 1st.
Replaces: This benefit replaces the summon familiar class feature.
Benefit: You can spend 100 gp to perform a ritual dedicated to the deity of your choice and summon forth a spirit creature to aid you in your adventures. This creature is both invisible and intangible, but it provides you with real benefits.
The divine companion stores arcane energy and redirects it to you according to your level and your needs. To fill this reservoir, you must cast one or more targeted spells at this creature. A spell so cast does not produce its normal effect; the companion simply absorbs the spell's energy. It can store a number of spell levels equal to your arcane caster level. You can cast spells into it at any time, but after you rest and recover your spell slots for the day, the divine companion loses any stored energy.
The divine companion exists outside reality and cannot be affected by spells, spell-like abilities, or any sort of antimagic effect or dispel check. It cannot, however, release its arcane energy within an antimagic field or similar effect. It uses your caster level for any checks required, and when it releases arcane energy, the level of the effect created equals the number of spell levels released.
The divine companion can release its stored spell energy in either of the following ways, at your command. You can use each effect as often as you wish, until the companion's stored energy is depleted.
Healing (Su): As a standard action, you can order your divine companion to convert the energy it has stored into healing power. This is the equivalent of a conjuration (healing) spell. This effect heals you for 1d6 points of damage per stored spell level, or as many levels as you designate (up to the maximum currently stored).
Shielding (Su): As a swift action, you can order the creature to convert the energy it has stored into protecting you. This is the equivalent of an abjuration spell that provides you with a deflection bonus to AC and a resistance bonus on saves each equal to the number of stored spell levels, or as many levels as you designate (up to the maximum currently stored). This effect lasts for 1 round per arcane caster level you possess.
This issue came up in my RL game a few months back. Here is the reply I gave them:
Did a little delving into the rules issues that came up last night.
Erring on the side of giving Ryan both bonuses for invisibility and flanking was correct. Invisible flanking is a slightly problematic area of the rules where the rules mechanics conflict with perceptions of why the rules are the way they are. There was even FAQ clarifications that had a stance counter to the rules, but that then creates its own problems.
Problem 1: As the rules are written in the book, Joe the hero could be walking through a field and get approached by Thog the orc. "Thog walks up and sneak attacks you, Joe," the DM says. "How is he sneak attacking me?!?!" Joe asks, incredulously. "Well, Goht the orc is standing on the other side of you, invisible, and granted flanking to Thog by threatening you with a rusty butter knife." "But Goht didn't distract me in any way from focusing my defense against Thog; I didn't even know he was there!"
The "fix" to this issue, which was suggested in the 3.5 FAQ (but was later removed from the FAQ), is that invisible opponents don't provide flanking bonuses. This leads to Problem 2 (which is what you all dissuaded me from last night).
Problem 2: Randy the Rogue and Izzy the Improved Invisible rogue attack Stupid the Wizard in a well-lit alley. They chase down Stupid after two rounds of running (nobody is flatfooted anymore) and both rogues attack in round 3 of combat. They have moved into flanking position but only Izzy gets sneak attack damage (since Izzy is invisible) while Randy gets no sneak attack. However, if Izzy had NOT been invisible, both Randy and Izzy would get sneak attack damage (because they could flank). So it is better to be attacked by two unbuffed visible rogues than one invisible and one visible rogue! Which seems to go against "common sense," given that invisible opponents should make things tougher.
Problem 2 doesn't actually exist until you alter the rules to try and fix Problem 1.
On top of all that we get to add the craziness of Uncanny Dodge and scent.
So, all that to say, an invisible attacker can flank and can help provide flanking for an ally, but realize that you may get mysteriously flanked by an invisible orc with a butter knife.