Disclaimer: My Koine Greek skills are very basic so I welcome any knowledgeable critique of my observations below.

It appears to me that the NIV translation supplied (along with most if not all English translations) misses the meaning of the passage and instead conforms it to Trinitarian dogma. "PNEUMA" ("spirit/breath" (which are synonyms)) is a feminine noun. The use of the Neuter form indicates to me that this should be translated along these lines:

"...in which ye were sealed with the holy "breath of the promise..."

In other words "holy" does not modify the word "spirit/breath" but the phrase "breath of the promise"; otherwise it would be in the feminine.

So what is the "breath of the promise"? The "breath" is inextricably part and parcel of the promise. That is, proclamation of the message of the life, death, resurrection and ascension of Jesus is made by breath:

Weymouth New Testament John 6:63 It is the spirit (breath) which gives Life. The flesh confers no benefit whatever. The words I have spoken to you are spirit (breath) and are Life.

New International Version 2 Cor 3:6 He has made us competent as ministers of a new covenant--not of the letter but of the Spirit (breath); for the letter kills, but the Spirit (breath) gives life.

The notion that breath gives life is not original to either John or Paul (or Jesus) but is an "elementary principle" of the scriptures:

New International Version Genesis 2:7 Then the LORD God formed a man from the dust of the ground and breathed into his nostrils the breath of life, and the man became a living being.

Paul refers to this as "the principle of the breath of life":

BSB Romans 8: 1Therefore there is now no condemnation [death sentence] for those who are in Christ Jesus. 2For in Christ Jesus the law of the Spirit [breath] of life has set you free from the law of sin and death.

So not only do Koine grammar and deeply entrenched scriptural principles argue for the message being in view, so does the immediate context:

NIV Eph 1: 13And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him [by it] with a seal, the promised Holy Spirit [holy breath of the promise], 14who [which] is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.

So no, Eph 1:13 is, as I see it, certainly not identifying "the Holy Spirit" as a person. That it appears so in English translations is the result of eisegesis, not an exposition of the Koine, the immediate context or the "pneumatology" of the scriptures.

Disclaimer: My Koine Greek skills are very basic so I welcome any knowledgeable critique of my observations below.

It appears to me that the NIV translation supplied (along with most if not all English translations) misses the meaning of the passage and instead conforms it to Trinitarian dogma. "PNEUMA" ("spirit/breath" (which are synonyms)) is a neuter noun which allows it to be translated along these lines:

"...in which ye were sealed with the holy breath of the promise..."

So what is the "breath of the promise"? The "breath" is inextricably part and parcel of the promise. That is, proclamation of the message of the life, death, resurrection and ascension of Jesus is made by breath:

Weymouth New Testament John 6:63 It is the spirit (breath) which gives Life. The flesh confers no benefit whatever. The words I have spoken to you are spirit (breath) and are Life.

New International Version 2 Cor 3:6 He has made us competent as ministers of a new covenant--not of the letter but of the Spirit (breath); for the letter kills, but the Spirit (breath) gives life.

The notion that breath gives life is not original to either John or Paul (or Jesus) but is an "elementary principle" of the scriptures:

New International Version Genesis 2:7 Then the LORD God formed a man from the dust of the ground and breathed into his nostrils the breath of life, and the man became a living being.

Paul refers to this as "the principle of the breath of life":

BSB Romans 8: 1Therefore there is now no condemnation [death sentence] for those who are in Christ Jesus. 2For in Christ Jesus the law of the Spirit [breath] of life has set you free from the law of sin and death.

So not only do Koine grammar and deeply entrenched scriptural principles argue for the message being in view, so does the immediate context:

NIV Eph 1: 13And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him [by it] with a seal, the promised Holy Spirit [holy breath of the promise], 14who [which] is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.

So no, Eph 1:13 is, as I see it, certainly not identifying "the Holy Spirit" as a person. That it appears so in English translations is the result of eisegesis, not an exposition of the Koine, the immediate context or the "pneumatology" of the scriptures.

Update:

Please see my answer to this related post for what is meant by "sealed" which is actually referring to an "inscription": In Eph. 1:13, what does the word "sealed" (ἐσφραγίσθητε) mean?